I think you'll be out of luck at Boscolo Rome. I tried several dummy bookings for 3 and no dice. Go with the Grand Flora; the only other alternative is the Rome Marriott Park Hotel which appears to be quite a distance from city center.

In Venice, I recommend that you go with another hotel chain. The only Marriott hotel I'd consider there is the Boscolo Venice; all others are NOT where you want to be. And Boscolo Venice doesn't accomodate 3 in a room.

Or you can bite the bullet and get two rooms.