I think the OP is asking space wise is enough. And yes 3 slots are spaced enough that you can go Crossfire without a problem.
or.. asking if 3x 16x is enough. Which is also a yes. Unless you are on the X99 or X79 platform those PCIE slots will turn into 8X when more than 1 card is used. Doesn't matter since the bandwidth of PCIE 3.0 8x isn't fully used yet. Maybe in a generation or 2.