On the bathroom wet vents, going downstream from the dry vent, minimum sizes are as follows. (Tub/shower order doesn't matter, and there are different ways to a do a double lav.) 2" dry vent, 2x2x1-1/2x1-1/2" double fixture fitting for the two lavs, 2" lav drain, 2" horizontal wye for the tub (or one inlet is 1-1/2" if you use a 1-1/2" tub trap), 2" tub/lav drain, 3x2x2 wye for the shower, 3" shower/tub/lav drain, 3" wye for the WC, 3" bathroom branch drain.
If you don't need the vent connection at the double san-tee in the picture at the OP, there are a variety of ways to bring the two horizontal 3" branch drains together and connect them to the building drain. As seen from above, the two 3" branch drains are coming in at 3 o'clock and 9 o'clock, say; what angle does the building drain exit at?
Cheers, Wayne