I already beat the main game, do where do I find Reshiram, Zekrom, and Kyrem?
To get Reshiram/Zekrom you have to defeat it in a battle against N in N's Castle. It turns into the Light/Dark Stone, then you take it to Dragonspiral tower to catch it.
Kyurem can be caught in the same place as that fusion encounter where you are forced to defeat it. It drops the DNA Link item when captured, which will allow you to fuse it with Reshiram/Zekrom to turn it into Black Kyurem or White Kyurem.
We shall always remember the departed... White FC: 0862-2660-4156 / 3DS FC: 2492-4470-1418