Can I have three sanitary tees in the soil stack? How close can they be (vertically), and can lavatory and/or shower connect below the toilet?

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Philadelphia, Pennsylvania
Currently the toilet, lav, and bathtub drain through a single sanitary tee into the stack. It's not an ideal setup and is not vented. I'm replacing it and had planned on mostly matching the same layout but adding AAVs, but I want to consider another option.

Instead of one sanitary tee connected to the stack, I'm considering using three:

- one 4x3 tee that drains the toilet (about 2 ft total drain length to stack);
- one 4x2 san tee directly below that drains the tub (about 10 ft total drain, pipe would run below joist along wall to avoid cutting joists); (alternatively, I could omit this san tee and lower the 4x3 tee to pick up both of these lines, but it would require a larger soffit to cover the pipes)
- second 4x2 san tee about 12-30 inches below that to drain the lavatory (about 5 feet total pipe, runs within wall cavity).

My question is whether it is code compliant under IPC (and generally wise/prone to problems) to stack multiple sanitary tees on the same soil stack, and specifically whether there are requirements for the distance between any connections or the distance below the toilet connection?
Hey, wait a minute.

This is awkward, but...

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