https://forums.geforce.com/default/topic/469402/

In short, GPU and memory must be identical between cards.
And the sub model name such as "Ti" of "Geforce 1080 Ti" must also be identical.
The bridge connections of founder's edition and its own model of GigaByte are at the same position, hence, there would be no difficulties using SLI bridge, I guess.

And I found a comment that says NVIDIA officially supports only 2 Way SLI Bridge:

http://www.tomshardware.com/answers/id-3355119/geforce-gtx-1080-sli.html

And it is confirmed by the NVIDIA page:

https://www.geforce.com/nvidia-sli-bridges