Well, they did share over half their lives with her

Maybe I've been thinking about it wrong. While they all tolerated each other and occasionally managed a curious sniff (soon interrupted by batting of paws on both sides), I never really saw them as friends.

Daisy and Monkey seemed to have a kind of timeshare going on with sleeping spots and lap time, and Daisy and Elbie would occasionally be found asleep on the bed at the same time (not cuddled up, though, and generally when Elbie woke up and noticed she was there, he would leave). Occasionally they'd let us cuddle two at the same time.

But when you think about the age when we got the boys (1.5 years) and the age they are now (3 years & ~4 months), they've shared a bit over half their lives with her, and there's a big possibility that they are actually grieving too, in their own ways. Extra cuddles are already given.