add shower DWV

I need to install a new shower in a 2 story house with a concrete slab floor.
(no houses here are post tension)

There is room for a 36x36 shower at a pantry which has a common wall with a half bath.

Question- Assuming ( because I can only assume at this point) the existing toilet has a 3'' drain and a 2'' vent can I tie the new 2'' shower drain to the 3'' and a
1 1/2 vent to the 2'' vent? The vanity must also be already hooked up to this 3'' somehow.

Thanks

hummmmmmmmm,(clearing throat) yes

Thats kind of how I feel!!
I think I'll jack hammer out the floor 1st and see which way the drain from the toilet is running,I can't go out into the next room. If not enough room replace floor and reset toilet.

Thanks for the vote of confidence.

toilet

The drain from the toilet should go back and then down and you have to connect to the "down" portion.