SLI GTX 980ti's run at x8 x8 on that motherboard, not x16 x16, and that is enough to limit them somewhat.
The third x16 slot, will run the PCI-e SSD, but I do not think it has x4 available with SLI, and the sound card will fit in a PCI-e x1 slot, if that is all it needs.
Lastly, I assume that your GPUs are thin enough to provide physical access to the PCI-e x1 slots between or beside them.