Can I offset my stack horizontally in this situation?

asktom and/or Terry-
Just to be clear on the venting:
-I have a 3" drain for the toilet which runs under the slab to the 3" vent we have been discussing; this vent and this drain are 4' apart.
-I have a 1-1/2" vent over by the separate shower drain; this vent is about 2' from the 3" vent.

I'd like to summarize what I think I understand from what you both have written: once I bring the 3" vent up with 45's under the sink, I can tie in the sink's P-trap to a sanitary tee, then reduce the 3" vent to 2", bring it up inside the wall, tee-in the 1-1/2' shower vent to the 2" vent at least 6" above the sink's flood level, and then continue the 2" vent up through the roof.

Sorry to beat this to death, but I'm a belt and suspenders type and want to make sure I haven't misconstrued the info you provided. Thanks.

You need a 3" clean-out and then you can reduce to 2". There is more than one way to do that, Terry and I gave a couple ways above. (I started to write out options but there are too many - just make sure the drain doesn't end up too high). The rest of what you said is correct.

asktom-
I am putting in the 3" cleanout between the 45's...just didn't mention it for simplicity. And I have enough room for the sink drain to be positioned properly as well. Thanks for your patience and helpfulness.

You can reduce at the top of the clean-out and use a 2" 45 to turn up.

asktom-
Thanks for pointing that out. Now I can avoid having to fir out my wall to accommodate the 4" outside diameter of the fittings that 3" pipe takes.

Ok...I've got all of the venting done as you guys recommended. I have one last question (I hope). The tailpiece for my sink is 1-1/4" and the san tee is a 2 X 2 X 1-1/2. Is there any requirement (or benefit) for the P trap to be 1-1/2", or can I just go from the 1-1/4" tailpiece to a 1-1/4" P trap and then to an adapter to connect to the 1-1/2" san tee?

I pick up 1.5" p-traps that come with the 1.5 x 1.25" reducing slip joint for the 1.25" tailpiece.
You can also use 1.25 p-traps, but they are harder to find. I hardly ever use one.

Thanks, again, Terry. When I went looking locally, I could only find the 1-1/4" P-trap, and I wasn't sure about what configurations existed for a 1-1/2" P-trap.