You can't wet-vent a fixture (shower) downstream from a WC. The 3" vent changes from vent to drain where the sink connects.
The sink and the WC are "deemed" vented, and the WC is wet vented thru the 3" stack. Not sure what the 2" vent line does... unless it continues down to the lower level(s). If not, you could probaly connect your shower drain into that. Else, use it to vent your shower.
You can only use a san-tee to change from horizontal to vertical in a drain. (someone pls correct me if I'm wrong)
Not impressed by the 1/4 turn of the 3" stack (where it changes form vert to hor). Is it a LS (long)?