I'm building a 2nd-story bathroom with a toilet, sink and shower/tub. They all connect to a 3-inch horizontal branch with a 2" vent pipe that runs outside and up the side of the house. The vent pipe connects nearest the toilet.

Is this enough vent for the 3 fixtures?

Keep in mind that the vent was active before you added the extra bath. 2" is not the norm, I'd go 4" up through the roof.

I've posted a picture to clarify. The vent actually only serves this bathroom, and I'm just replacing an existing bathroom, not adding it. The previous owner had installed one of those air admittance valves above the sink, but I want to avoid that if possible.

This diagram looks down on the system, so it doesn't show the elevation drop, but there's plenty of drop in the system.

The vent goes out the side of the house and runs up past the roof. Weird, I know, but there it is. Adding another vent wouldn't be easy, but I might be able to do it if necessary. The bathroom is small, about 4.5'x6.5'.

Do I need more vent power?