WARRIORS OROCHI 3 Ultimate

One of the techniques that can be done in this game is the triple rush. To enact this, hit the character switch after your active character connects with a charge attack (any attack generated from the Y button, either standalone or finishing a combo). This will cause one of your allies to jump in with a rush attack. As that happens, switch to the third character for another rush attack, and then finally back to your starting character for the third rush attack to finish up the triple rush.

In this game, there are certain 3-warrior combos that will trigger a much more cinematic and devastating rush attack. For this achievement, you need to have used all 60 of those.

Each of the combos needs a particular warrior to be the trigger. If you do the standard "enemies knocked up, swap, slash in mid-air, swap, and smack into the ground" animation, it's not gonna count. You also need to get at least on kill with the Triple Rush, so you can't do it in the middle of a barren field.

For a complete list (Credit to the fine folks at the Koei Wiki):

1) Xiahou Yuan, Xiahou Dun, Xiahou Ba
2) Cao Ren, Dian Wei, Xu Zhu
3) Zhang Liao, Xu Huang, Zhang He
4) Zhou Yu, Taishi Ci, Sun Ce
5) Sun Shangxiang, Sun Ce, Sun Quan
6) Gan Ning, Ling Tong, Lu Meng
7) Zhang Fei, Guan Yu, Liu Bei
8) Guan Suo, Guan Ping, Guan Yu
9) Ma Chao, Huang Zhong, Zhao Yun
10) Xu Shu, Zhuge Liang, Pang Tong
11) Sima Shi, Sima Zhao, Sima Yi
12) Ma Dai, Pang De, Ma Chao
13) Wei Yan, Meng Huo, Zhurong
14) Lu Xun, Cao Pi, Jiang Wei
15) Zhenji, Yueying, Lianshi
16) Deng Ai, Meng Huo, Huang Gai
17) Keiji Maeda, Toshiie Maeda, Katsuie Shibata
18) Kanetsugu Naoe, Aya, Kenshin Uesugi
19) Yukimura Sanada, Kunoichi, Shingen Takeda
20) Kiyomasa Kato, Masanori Fukushima, Mitsunari Ishida
21) Yoshihiro Shimazu, Ginchiyo Tachibana, Muneshige Tachibana
22) Kenshin Uesugi, Shingen Takeda, Ujiyasu Hojo
23) Nagamasa Azai, Muneshige Tachibana, Hideyoshi Toyotomi
24) Muneshige Tachibana, Tadakatsu Honda, Keiji Maeda
25) Cai Wenji, Zhenji, Motochika Chosokabe
26) Daqiao, Mitsunari Ishida, Xiaoqiao
27) Xingcai, Ina, Guo Jia
28) Daqiao, Himiko, Bao Sanniang
29) Zhuge Dan, Sima Zhao, Sakon Shima
30) Dong Zhuo, Da Ji, Orochi
31) Yoshimoto Imagawa, Zhong Hui, Yuan Shao
32) Meng Huo, Ding Feng, Kotaro Fuma
33) Zhurong, Nene, Wang Yuanji
34) Goemon Ishikawa, Hideyoshi Toyotomi, Ling Tong
35) Zhang Jiao, Magoichi Saika, Gracia
36) Ranmaru Mori, Mitsuhide Akechi, Zhou Tai
37) No, Wang Yi, Oichi
38) Liu Shan, Motochika Chosokabe, Yoshimoto Imagawa
39) Kojiro Sasaki, Musashi Miyamoto, Xu Huang
40) Hanbei Takenaka, Jia Xu, Kanbei Kuroda
41) Hanzo Hattori, Kotaro Fuma, Ryu Hayabusa
42) Da Ji, Motonari Mori, Achilles
43) Kasumi, Kunoichi, Ayane
44) Diaochan, Okuni, Sanzang
45) Lu Bu, Cyborg Nezha, Yoshitsune Minamoto
46) Zuo Ci, Kanetsugu Naoe, Seimei Abe
47) Nobunaga Oda, Cao Cao, Nemea
48) Liu Bei, Ieyasu Tokugawa, Joan of Arc
49) Guo Huai, Masamune Date, Rachel
50) Sun Shangxiang, Kai, Sterkenburg Cranach
51) Sun Jian, Nene, Sophitia Alexandra
52) Ayane, Rachel, Momiji
53) Fu Xi, Nuwa, Shennong
54) Benkei, Yoshitsune Minamoto, Kiyomori Taira
55) Kiyomori Taira, Orochi X, Sun Wukong
56) Shuten Doji, Taigong Wang, Kaguya
57) Da Ji, Seimei Abe, Mae Tamamo
58) Susano'o, Yinglong, Human Nezha
59) Gyuki, Dodomeki, Hundun
60) Hundun, Kyubi no Kitsune, Orochi X

All great solutions here, but what I found myself searching for was "how can I do them better", so here's a list of things the other guides haven't covered that I found on my own.

1. There are no Items you can equip that will help.
2. The "Trinity" character ability increases your True Triple gauge, but is pointless as you won't be able to include them in your roster.
3. The attacks must be done in a specific order.
4. You must build your Triple Rush gauge before you can attack. This is the blue/green bar in the bottom-left, curves up below your Musou bar. There is a sound when you hit the limit.
5. Don't die!

So, here's how I did it :

1. Fire up Story Mode
2. Go to Battle Preparations / Change Team
3. Choose your 3 characters IN ORDER listed in EmporerDragon's guide above.
4. Using his first entry as my example, your last character should be Xiahou Ba. Press to accept.
5. Options / Save. This is very important. I like to do it after choosing my characters so its all ready to go if you need to reload.
6. Begin Advance, pick whatever stage you want, I've been using Rescue at Yiling out of habit. Don't choose Easy, pick at least Normal, depending on how good your characters are. For Level 1 guys I choose Normal, anything better and I do Hard.
7. Skip chat ( x2), Begin Battle ( x2). After loading, hit A then the 3 crack lines again to skip the intro movie.
8. Now wail away on enemies. Use your lowest level/weakest character you have, as the True Triple gauge seems to go up based on the amount of times you hit an enemy. Not number of kills, combo or anything. More hits on the first group of enemies, the better. It is very much possible to have a full Triple Rush gauge with only 10 kills. This means less running around to get kills, this is why you don't choose Easy difficulty. On hard with a sub-100 character you should be able to max the gauge on the very first group of enemies, if all of your characters are maxxed/Mystic, you could always buy a 1-star weapon from the blacksmith. Either that or spend 5+ minutes on the stage clearing out peasants.
Update: You can skip this, Triple Rush will use Musou, cheers Saracin
9. Once you PING and your True Triple gauge is maxxed, jump off Red Hare (as if there is any other mount), change to the third character in the roster (Xiahou Ba) and hit your button.
10. Wait for the enemy to flash yellow when they're hit, then MASH the Right Trigger . Don't stop mashing until all 3 characters have had a hit and you're back to your first one (Xiahou Ba). You do not need to time these hits, mash that button as fast as you can if you like.
11. Once the sequence is complete, you won't have any popup or any kind of register that you've done the move, so press to pause, hit the button then go to the left and to the Achievement app thing. Not the snap, the new one that they added. I guess snap would work but I don't like it.
12. Scroll down to the secret achievement for this. It'll probably be at 1% after your first True Triple Rush. Hit to make it your favourite so it appears at the top of the list from now on. If it isnt 1%, you did something wrong, possibly the order wasn't right. Go back to the game and try again. Your Triple Rush meter will still be full, ready to go again.
13. If the completion % has gone up, then you've done it! Go back to the game and choose Return to Camp. Do NOT under any circumstances choose Quit otherwise your progress will be reversed. Yes, they can undo achievement progress.
14. Once back at camp, either go back to Step 2, pick the next 3 characters and continue, OR go to Options and Save your game before quitting.

** If you die, your progress will be reversed to your last save ** Always save your game before starting a stage. If you don't and you get through 5 or 6 triple rush moves, returning to camp after each one, then you die, all that progress will be reversed the next time you do a Triple Rush move!! If you think you are going to die and you haven't saved in a while, you can make use of the interim save, however keep in mind that if you die and have to resume, you lose the ability to return to the camp. You will need to see the stage all the way through to retain your progress.

Follow EmporerDragon's list to the letter and you should have this. I just wanted to include my specific steps, confirm it works without having to complete the stage (just don't "quit"), and confirm that there are no other items or anything else that will help this achievement.

For those that it glitched on, I don't have any fixes other than those listed above. Can only imagine that after doing the move, you died or quit instead of returning to camp and that progress was lost. Only the "return to camp" option keeps your achievement progress.

Good luck.

EmporerDragon has a great solution, but for some the achievement may glitch out due to how the game requires the user to sign-in during the game. If the glitch happens and you are stuck at 98-99%, you can still get the achievement, but it will require some major replaying. To get the achievement, you must delete your save file from all sources (restarting the game from scratch) and must unlock all the characters again. Deleting the save file resets the progress of the achievement in the Achievement App, which makes you have to redo all the combos again, and since the save file is deleted, most of the characters are locked. Hopefully this doesn't happen to anyone like it did to me, but this is a decent fix if you are a completionist.