Are the piggies all living in pairs/group now? If there are any piggies needing to live alone, their cages must be side by side with another piggy(s) so they can have interaction through the bars. You can’t keep single piggies in stacked cages as they can’t interact and will get lonely.
The overall size is of course going to be dependent on how many piggies you will have in each cage.
If you are going to make it three tiers, then I think allowing for height in each level, plus a gap between each level to allow access to each cage, it could end up too high to be manageable and easy to clean the top cage. Just something to think about!