Actually, this is something most people really get confused on (and I did too until I really delved into it for a bit). As what Dr. S recommends, P3 starts after 72 hours after the last dose (so not after the entire day of your last dose, but literally your 72 hours starts the moment you take your last dose of hCG). It should be noted that we are counting by the hours, which means if your last dose is TUESDAY 8am for example, add in 72 hours = FRIDAY 8am.
I have a detailed article and a cool little table to show you when P3 starts depending on what day of the week your last injection/dose of hcg was here for you: