My question must have been too involved. I am going to replace all the pipe now.
I spoke with the plumbing inspector and he gave me the go-ahead to replace all the pipe at the current depth. He said even if the code now says 12" below slab, it is not something they require when replacing existing pipe. He recommended 3" schedule 40 to the 3rd toilet and 4" from there to the building exit. I read that only 3" is required for 3 toilets, but I guess that gives us some leeway for the future.