If I have taken 3 different creatures with 3 seperate {Illusory Gains}, and my opponent plays 1 creature, do all 3 of my Gains go on that one creature, thus letting my opponent have all 3 of the other creatures back?
It appears so.
Yup. Each gain will trigger upon entry of a creature. It does not matter the creature is already under your control once the trigger resolves.
So play it like a legendary enchantment. Hmm
For completeness sake:
25-2-2015 If multiple creatures enter the battlefield under an opponent's control at the same time, Illusory Gains will trigger for each of them. You may put these abilities on the stack in any order. As each resolves, you'll briefly gain control of the associated creature. The last one to resolve determines which one you'll ultimately control.
You choose the order of stacking for each gains, so if multiple creatures enter, you can end up with different creatures.
Interesting. If they drop three tokens in, I can stack em to land on all 3.