Sorry to revive an old post but thought some additional comment might help others looking into GPU rendering.
SLI has nothing to do with GPU rendering. It’s a gaming feature. Still, a board with three way SLI means your setup can handle three full-size GPUs. In that sense, SLI support can be a good gauge for a mainboards capabilities.
Redshift can use each GPU to its full potential memory-wise. Memory isn’t combined but it also is not constrained by your lowest card. Other GPU renders are often less accommodating than Redshift.
Consider that for GPU rendering you want one card to drive your display and one or more other cards exclusively for rendering. I use a modest 750ti for my display and a pair of 1080s to render.
Lastly, get as much RAM as you can. You definitely need more RAM than the total combined RAM in your GPUs and make sure that your system and CPU have enough PCIe lanes to run all of your GPUs. Some CPUs have limited PCIe lanes which could force multiple GPUs to run at reduced speeds which could throttle performance.