Do 3D programs benefit from multiple gpus?

uses for pc: 3ds max, maya, UDK, Unity (game engine) photoshop, some after effects and gaming too 
Gpu: nvidia brand , 660ti, 670, or 680

 Ok. Im about to purchase a new pc, I just have to figure out the gpu I'm getting.I just need to know if the programs I listed above benefit from multiple gpus. That would affect my build, i read that some programs take advantage of the Cuda cores in these gpus. So if in fact Sli does affect it, then I'm better off going eith 3x 660 tis vs a 680 or 690 even, as iitd be cheaper for me.I'm only buying one card at the moment and I'd buy another like every 3 weeks. Now, if it doesn't matter and multiple gpus won'. Makea Difference, then I'm better buying the single best card I can afford, if in fact sli is only suitable for gaming. Can someone please help me understand? I'm writing from a phonr here sorry for typos and thanks in advance. 

So quickly, does 2or3 x (gpu) =2or3x number of cuda core ls or does itnot work that way? 
For example, the 660ti has 1344 cuda cores while the 690 has 3000+(forgot exact number) so letts saye, 3x 660 ti @ 270 $ is $810 for 4000 cuda cores vs single 690 @ $1000+ and 3000 cuda cores.