Toilet plumbing installation

We are adding a story-and-a-half addition, with baths both up&down-stairs. I have my wet wall from the downstairs bathroom in a location which allows for the upstairs toilet to flush directly into a vertical 3" pipe, which I will vent with a 3x3x2 wye positioned approx 4-6" below the closet flange. The wye will aim for the nearby upstairs wall housing a 2" vent pipe.

Are there reasons to not do it this way? I think the layout may work better this way, however, I don't know if it's proper. I'd just ask the inspector, but I don't want to look retarded if there is a major reason for not straight piping this toilet. My original option was to run it the normal way, with the flange connected to a closet bend connected to 3" pipe, connected to a low-heel 90 w/2" vent, connected to the vertical 3" stack. Thanks much.

If that 3" pipe is the vent for the bathroom downstairs, you can't use it as the drain for the upstairs bathroom without relocating the vent for the room below. You'd have to run a new vent from downstairs up above the flood plane of the highest drain connected to it from upstairs (nominally 36"+6" in a kitchen, or whatever your vanity sink height is plus 6").

No, the 3" is not the drain from the downstairs toilet, I should have clarified that. The 3" was my rough-in out of the concrete slab, destined to be the drain for upstairs. It is however, the only thing on that drain. Is that a bad thing? I've read that having another fixture drain into a soil drain is good method of "washing" that drain. Nothing else drains into this 3" pipe until after it enters the slab, where the downstairs sink and laundry branch hook into it. Thanks.

Look "retarded" I say. The inspector might respect that more and even aid in giving some sound advice.