There are already laptops with two desktop GeForce GTX 1080s. If you're willing to put up with the weight of a cooling system, I don't think tri-SLI presents an insurmountable problem.
And if you're looking at a three-screen prototype, weight is certainly not a problem.
Of course, they might not have gaming-grade GPUs in them at all. Driving three 4k screens in 2D isn't that difficult and that's all you'd need for a demo unit.