This is Charlie, Tuppence and Lottie's 4 level cage. Charlie and Tuppy have the bottom 2 levels, while Lottie has the top 2.

If the three of them get on at some point in the future, all we need to do is remove one grid to allow access to all the level for the piggies. There will probably be four piggies by then, as Lottie will get a friend just in case Charlie & Tuppy don't accept her again.