Actually, macs have been on Buffy since the start, I believe. Willow totes a powerbook, and it featured heavily last season when she was trying to break Adam's code.
Oops, that may have been spoiler, as you've only just got season 3... sorry!
Season 4 continues to rock...
Angel is not so blatant about Mac use. Cordelia bought a cheap PC for the office... that's what it looks like and she's sooo cheap-er, thrifty. However, some of the closeups have appeared similar to MacOS, so perhaps they're faking the thrifty PC bit and there's a gleaming G4 hidden beneath the beige.
For what it's worth, any of the monsters on Buffy that have been computer savvy have appeared to use PCs. (Adam, definitely.)