Have you checked out cavycages (guineapigcages.com)? They give minimum sizes for piggies and for 3 piggies they suggest 2 grids wide by 5 long. You can then add on a hayloft or eating area etc. In a pack you usually get quite a few grids, can't remember how many, but certainly enough for 2 x 5 plus extras. I haven't used the connectors that come in the packs I used cable ties from diy shops, I struggled with the connectors to be honest.
