ReCore

Hey! Both solutions so far explain this perfectly, but I recorded the little clip of when I got the achievement just as a frame of reference for anyone still stuck!

Head over to the area right outside of Joule's Crawler in the Granite Steps, where you'll find 3 Spiderbots level 2-3. Damage them with either RT or the T8NK as mentioned by Aeons of Blight, while continuously calling your corebot to you using the X button.

With any luck, you'll manage it like I do in the clip! This actually took me a couple of hours, as I seemed to have done it a couple of times with no pop! Hopeless commentary, but I hope the video does help!

1. There are two groups of enemies(LV2~3) near the base of Joule, a group of blue, another
group of red, choose "red group"
2. Use RT to weak the enemy(don't kill anyone, just Leave a little bit of energy), then hold RB to charge(Split Shot) and aim 3 enemies.
3. Make sure they are not on the same trajectory, then fire.
**(In the process, keep to input X button, call your robot back, don't let it to kill the enemy)

If you're high level like me, and you have the issue of your bots killing everything, this tip is for you.

There are 3 blue spider bots outside to the left of Joules Crawler. Use the new Tank Core Bot and ride him. Use the charging attack (B) to ram them a couple times till their health is low. Hold X to jump off the Tank and hold RT to target them all and they should all die with ease.

This eliminates the possibility of your corebots killing off any of the enemies and makes it a while lot easier. I did this on my first try with this idea. Hope it helps

This is the easiest method i have discovered, you can get the achievement in first try.

Right when you exit Joules crawler, there are 6 crawlers near by (3 Blue & 3 Red) . Get 5 of them together and shoot when at least 3 of them are at equal distance from you, because split shot travels slow and uneven distance can create a delay greater than 1 second. Always remember to keep spamming to keep your core bots from attacking the spiders. This method is far easier than trying only with 3 spiders. I got this in my first try.

This solution is for the ones highly leveled up and the strategy used is on Granite Steps just outside Joule's crawler.

I'm level 40 so that means my rifle is fully maxed up as well as my frames. Plus all my frames have the best available parts equiped so any of my three frames kill the spider bots outside Joule's crawler in a second. But following the suggestion on Darth Barthus' solution I did something similar that worked perfectly and I got the achievement on my first attempt.

What I did was to create a basic spider frame instead of an ape and put Mack on it (blue core).

As soon as you leave the crawler charge your shot and hold it, when you spot the 3 red spiders outside the crawler get a little close to them but not too much or they will start chasing you and get off the lined up formation.

Use Mack spider frame's lethal attack on them. When Mack is inside the spider frame it shoots a couple lasers with a long and wide range. Since the 3 spiders are close to one another any one you lock on the laser beams will stun all 3 and leave them in critical condition but alive, just enough for you to deliver the killing blow so immediately after you stun release the shot and hopefully they'll be lined up in a perfect position for your shot kill them all at once.

Good Luck!

Using the three solutions above I would get close but my bot kept interfering. I made a basic ape frame put Seth into it and when I left the crawler I sent him after the red enemies off to the right. He was gone long enough to get the triple kill.

I found the easiest way is at the end of the well of shadows dungeon. After defeating some enemies behind barriers using the lob shot, groups of three enemies will fall from the ceiling. The first is red, the second is yellow, and the third is a mixture of all three colors. They are pretty easy to take down so I didn't weaken them much before using the split shot, but if you do be sure that all their health is not low enough that your corebot could take one of them out. You need all three from one the groups to get the achievement. :)