This collage was made by WOLFBLADE111 on Deviantart (Check out his stuff, he's awesome).
Which three characters can you summon? Also, special items will not be summoned along with its user, like choosing Thanos, but he won't show up with the Infinity Gauntlet. To prevent any mental breakdown, if you explain them how they were fictional characters, they won't care about it. If you wish to make things more interesting, instead of living on Earth, you and your characters can be taken to the ''Crossover-Verse''. This is a reality similar to Battleworld from Marvel Comics, an amalgam of worlds fused into a massive planet, so everyone has enough space to live and use their powers.
For example, I choose Kthanid, Android 21, Power Girl and Doctor Doom.