Pinball FX2

There are 2 ways to get heroes to join you. One is to hit the ball up the unite ramp. ( It says Unite under it) Each time it goes up the ramp it lights 1 letter. Once you spell Unite, the Letters will be blinking. Hit it up there 1 more time and you'll notice 7 ramps will be lit and all the names in the middle of the table will be blinking. Hitting a lit ramp will result in that hero joining. this is timed and there isn't much time, like 10 seconds.
Then you must do this a further 2 times for the achievement. Once you hit a ramp to get a hero to join you that lane will no longer light upon relighting Unite so you will need to hit 3 different ramps to get this.
A second way to get a hero to join is to hit the storm ramp. (it says storm at the entrance) The ball must go all the way around and swirl around the cup at the top of the left flipper. You'll notice on the dot matrix it will say Xavier’s
Gift and some of the letters will light up. It's takes about 3 times around to light up Xavier’s Gift. You must then hit it up the Unite ramp once. Then 1 random ramp will light up. You must hit that ramp to get that hero to join, however you have much longer to hit that ramp, like 30 seconds.
The easiest way to hit this storm ramp is with the upper left flipper. This random ramp that gets lit will always be one that you need, it doesn't light up an already lit ramp.
This will also reset the Unite letters so if you are close to spelling Unite, this resets so pay attention.
Not too hard overall.
Edit: Found a third way to collect a hero, with a Bioscan award. Hit the ball up the Colossus ramp 4 times to make a Bioscan award available. Then a late shot from the upper left flipper to hit the hidden sinkhole, also where you'd start the Magneto mission. This will return the ball to the left hand launch lane. It is pretty hard, but if you launch the ball correctly, where it stops just after the blinking light, you'll get a random award. One is a hero. You could also get an extra ball, another is lighting one of the letters of XMEN. A failed shot will light several letters on Xavier's award.

You have two options of getting heroes to assemble. I'll show you one of each in my video.

1) Hit the ball up the left ramp until "UNITE" is spelled out. Then hit it up once more. Now, every loop, mini-loop, orbit, and ramp will become a target at which you can recruit a hero. The only limitation is that you can't recruit the same hero more than once until you get them all, but you only need three for this achievement. This is likely the easiest way to obtain a team of three, as the options are more wide open.

2) Hit the ball with the left slingshot into the Professor X dome a few times until the words Xavier's Gift on the dot matrix are lit. Then, hit the ball up the left "UNITE" ramp. At this point, you'll be given a specific hero you need to go after, coordinating with the blinking lane. After playing this table enough, you'll know exactly which lane you need to aim at simply by which hero is selected. This one's a bit tougher because the hero is chosen for you. For example, Storm is a tougher shot than Jean Grey, so the first option gives you the choice of which to go after as opposed to this one.

I made this video.