Yes, IPC allows the shower to come in downstream of the WC. If the picture is showing the WC on the left, the lav coming in, and a turn down into the lower horizontal drain (running up/down the page), looks tight for adding another wye for the shower.
Bear in mind: the new shower trap arm runs from the trap outlet to the wye where it joins the lav (and WC) drain. That trap arm has to fall at least 1/4" per foot, but the total fall is limited to one nominal pipe diameter. So if its's a 1-1/2" tub drain, that can go at most 6', if you get the slope at the exact minimum Or for a 2" drain, 8'.
Cheers, Wayne