Progress.
As of late last night, all 3 cats are living on the same floor. The girls are still ostracizing Cesare, but Zia's approaching him now and Chiaro isn't flying for the basement. So at least I can make sure that everybody has eaten/drank/used the litter box. Phew! Now if only the girls (especially Chi) would stop hissing and growling at him. He doesn't understand why they're so upset (and he's being a super good boy about backing off when they hiss) and he wants to play with them, especially Chi, who is usually his playmate. She's just not having it yet. I tried getting them to play together with a feather last night but they stayed in their respective corners. Maybe it'll go better tonight. If everyone's friends again, I'll be a lot less stressed. ;)