Hi, just reading through some catch up's and not sure if you're still checking on here. But I wouldn't recommend stacking 3. I have a frame, then cage and one cage on top and that's about as much as the frame and the cage underneath can take, including the 3 heaviest piggies in the top cage plus housing stuff like wooden stick runs etc. Mine popcorn and bounce around quite a bit, especially when they here me washing their veg and I can see the thing bounce a little. It's safe, but as I say, 2 is as much as I can do.
Hope that helps.