Colosseum, Sistine Chapel, and St. Peter’s Basilica all in one afternoon?

You will have to walk through the very crowded Vatican Museums to reach the even more crowded Sistine Chapel. If I were you, I would skip the inside of the Colosseum, and focus my time and energy on the Vatican sights you want to see.

You have to walk all the way thru the Vatican Museums to get to the Sistine Chapel.
I can't imagine how you could fit all 3 in just a few hours.- Colosseum is not at all near the Vatican or St. Peter's
The walk from Vatican/Sistine to St. Peter's entrance takes a good 20-30 min alone.
St. Peter's will have a wait.
You will need prebooked timed entry for Vatican/Sistine and Colosseum.

Pick one.

Reading the itinerary it says only a walk around the outside of the Colosseum is included, and the day ends at the Pantheon. The Vatican Museum closes at 8pm. So some rushing around it may be possible to do all 3 in this order:
1. Backtrack to the Colosseum
2. Get all the way across town for St Peter's
3. Enter the Vatican Museum at 6 pm
It's not an afternoon, but it could work.

There's no ticket required for entry to St. Peter's, but there is a security line. A long security line, even outside of Holy Year. You could possibly spend hours in that line. There's only one way around that line that I'm aware of: Often commercial tours covering both the Vatican Museums/Sistine Chapel and St. Peter's Basilica are allowed to use a connecting door from the Sistine Chapel into the church. That allow their clients to avoid the long walk back through the museums to the exit, the long walk around the outside of the building to the Basilica and the very long security line. I don't think anyone can absolutely guarantee that a particular tour group will be allowed through that door, but it seems usually to work that way. I've seen no reports of a different procedure since the beginning of the year.

There is heavy demand for tickets and tours for the Vatican Museums. You should check availability for your travel date immediately.

What are the dates of your trip? I'm asking as it's possible they'll re-start night tours of the Colosseum this spring, so you could do the museums/Sistine/St Peter's in the afternoon, and the Colosseum at night. As said above, you have to pass through quite a bit of the Vatican Museums to get to the Sistine so while you could head directly there from the entrance, it's a long walk through a very busy place, and you can't exactly skip the museums altogether.

Optimally, to do it ALL would be to book an afternoon tour of the Vatican Museums + Basilica that specifically allows use of the inside passage between them (not on a Wednesday), and then do the Colosseum at night if they do indeed open up those tours for high season, and you're traveling during that time.

Alternately, you could try to pre-book the Colosseum for a time slot soon after your RS morning tour is over, and pre-book one of the very latest entrance times for the Vatican Museums (17:00 or 17:30). Have lunch in between. The museums don't close until 8:00 PM (although they start clearing the galleries/Sistine 1/2 hour before that). What this would not allow you to do, however is visit the basilica, as it closes at 7:00 (19:00) and you likely couldn't get through the museums to the Sistine, exit the museums, walk around to the basilica's entrance and get through the security queue before it closes.

You probably know but just in case.... the Vatican Museums/Sistine are closed on Sundays. :O)

Thanks to everyone for all of the great advice! We ended up booking a tour of the Vatican museums/Sistine Chapel/St. Peter’s so that we can avoid some of the lines. But I’m very curious about the night tours of the Colosseum - we’ll be there in mid-October. Does anyone know if they’ll still be running those then?

But I’m very curious about the night tours of the Colosseum - we’ll be
there in mid-October. Does anyone know if they’ll still be running
those then?

No. Not until they show up as bookable on the website.

We took an evening tour of the Colosseum back in November 2023. There have been a number of changes to offerings there in the interim, but at least they didn't limit those tours to warmer months in the past.

You probably want to do the Colosseum at the same time you visit the Forum and/or Nero's palace which are near by.

Doing St. Peter's and the Sistine Chapel will take up several hours, especially if you also visit Hadrian' Mausoleum which is between the Vatican and the Forum area.