The Preview does only loop once. It shows you one iteration of the layout, and then stops.
The loop option on a region is there for a tiny number of use cases that need it. In most scenarios, it shouldn’t be ticked anyway. Have a look at the FAQ on this and you’ll understand how the system works a bit better.
I can’t advise on 3x 4K videos as it’s not a configuration I’ve attempted personally. I’d imagine perhaps even and i5 might struggle, depending on how much hardware acceleration is possible with whatever graphics card you use for that.