I need advice! I'm Plumbing two back to back baths and I'm not quite sure how to vent/drain them. They are on the second floor and I'm running a 4" pvc drain up to the second floor. I'm installing a 4" sanitary cross below the second floor joists and coming out of both sides of the cross with 3" pipe and picking up the toilet and the tub (in that order). Out of the top of the cross I'm reducing down to 3" and bringing the two sinks into the 3" vent/drain at about 12" off the second floor. The 3" continues up to the roof.

Can I drain the tub upstream of the toilet into the 3" horizontal line? It's only abot 4' away from the toilet and the toilet is only 1.5' away from the 4" cross. I can add a 2" vent at the tub if need be. My concern is that most plumbing diagrams seem to indicate that the tubs and sinks should drain downstream of the toilets.

Can I also drain the sinks into the 3" main stack upstream of the toilets? The sinks are approximately 4' from the stack
Thanks for any help you can give me