Well, the Green was there first, alone for a few months. I added the Orange, it was not touching at first, it was mounted to a rock, so the Rock was sitting on the Green.
I really got lucky with the Purple. I was looking for a place to put it, it also was on a rock, and it just fit perfectly where it is.
It's probably a good idea to not just put them on top of each other, but let them slowly move into each other. When they do touch on the edge, they dont Burn and Die like other corals.
I'm getting some cool formations up where the Purple meets the Green.
Good Luck.