The main concept is a good one, but I found myself basically just solving each ball's maze one after the other instead of having to think about how each of movements was playing out in the other parts of the puzzle. I think it might make sense to scale down the levels a bit and make them more interconnected so that moving one ball actually has some consequences on the other ones (the spikes someone else mentioned are a good idea!). Increasing the time pressure could also be another one as someone else mentioned. I would also suggest maybe having switches or triggers that one ball could activate which would then open doors or get rid of traps that another ball would be safe to go through :)