3x GPU means you can have up to three GPUs in the system.
As for 4x, this is the speed of your GPU on the PCI Express bus. This means it is going to get 4 lanes (4x) of the PCI Express bus. For PCI Express 2.0 this means you will get 2GBps, for PCI Express 3.0 (rather new) you will get 4GBps.
For a single and dual GPU system, you want to be using 16x slots. Once you get to 3 and 4 GPU this isn't possible anymore in most systems. So you start running something like 8x, 8x, 4x. Where the first two cards are running at 8x, and the last card only gets 4 lanes. The 8x isn't a problem, in testing the performance difference between 16x and 8x is almost non-existent (sub 1%). Dropping down to 4x though may pose problems, especially with a 1080Ti.
I have no tested the 1080Ti or 4x performance, with PCI Express 3.0 it may even less of a problem as the bus speed doubled. I'm sure someone can do some testing when they have more 1080Ti's in the wild to what it means in raw performance.
If you think you are going to run 3 or 4 1080Ti's, I would highly suggest going with a Xeon build. This will allow a motherboard with 40 lanes.
A Z270 will allow 20 lanes, the older Z170 will only allow 16. The advantage of the Z270 is the ability to run two GPUs at 8x while still having 4x for an NVMe drive. Keep in mind, each NVMe drive will use up 4 PCI Express lanes.