I am building a bathroom suite in my addition & doing all plumbing myself. The shower is next to the toilet & I would like to know if I can do the following to eliminate installing an additional vent for the shower?: Install a 3x3x3-T w/2" side outlet just below the toilet. The 3" vent goes out the back of this. The 2" shower drain/trap is within 4' of this T & will go into the side. Can I use the 3" toilet vent as a common for both the 2" shower & 3" toilet drains? The toilet is direcly above the T. I looks logical & the information I have shows other configurations for common vents that are similar.

You are correct, it does look right and in my opinion would work like a charm.
However, All plumbing codes state that you may not discharge a major fixture, your toilet past a unvented minor one,your shower, and that's exactly what's happening here. Even though the toilet will vent at the same time the discharge passes the side outlet the fact that the toilet discharges above the inlet of the shower shoots your plan down. Now if there were some way to connect the shower into the lavatory drain it would be wet vented by it assuming the lav's vented. Regards, Tom