What day of the week will you be in Rome?
Level-setting for the Trevi Fountain visit: It is very picturesque, it is large and you'll be able to see it. But you will be joined by hundreds of other people. (I walked by it in February.)
The Sistine Chapel is part of the Vatican Museums. It's basically at the end of the walking route inside the huge complex, so it takes quite a long time to get there even if you already have a pre-booked entry. I booked online and got right into the building, but I still had to go through a line of 5+ people (again, in February) to swap my reservation document for a physical ticket--an annoying procedure common in Italy. The Chapel is what everyone wants to see, so it will be packed when you get there.
There's quite a long walk from the Chapel back around to St. Peter's, unless you're on a combination tour that gets to use the back door. Those are not short (or cheap) tours. If you're not on a combo tour, you'll have to line up for St. Peter's; that won't be quick. I don't know how long it's likely to take, because I haven't tackled it yet.
Ordinary tickets to the Vatican Museums are selling out way ahead of time. Online tickets for regular entry for most days in April are sold out, as are some days in May. When the online tickets are gone, your options are pricier tours or lining up outdoors, possibly for a very long time; you definitely don't have time to go through the line for those who haven't pre-booked an entry or a tour.
The areas you want to see (Vatican, Trevi Fountain and Colosseum) are not close together, so considerable time would be burned, just moving between them.
Rick has some good, free walking tours for Rome available on the Rick Steves' Europe app. I think the suggestion to settle for one indoor sight and walking around part of Central Rome to enjoy the city is a very good one.