Thanks so much for the replies and helping a DIYer work through the process...
I have since removed the shower and see that the vent is downstream of the shower drain. So HJ, you say if it's vented properly, 2" would be ok. Where should the 2" vent hit the 3" shower drain line for it to be enough? (BTW, I have already purchased the 3" drain line, so regardless of necessity, that's what I'll be using. granted, I probably won't need it, but I don't want a wading pool under any circumstances and I have 2 heads and 4 body sprays that might all be pumping at the same time, depending on how I set-up the diverters). And can someone explain the reason the horizontal run is bad in the first pic? Is that because it's upstream of the T/Y? because the way the shower is plumbed now, there is a horizontal downstream of the drain? The reason I ask, other than basic knowledge and clarification, is I could do a vent to the right of the tub drain, but it would have a horizontal run to tie in. If I do this, would I need to tie in downstream of the drain? And is there a limit to vent runs in this position? TIA again.