I'd be interested in seeing where you got that from.
Anya from Season 3 may not be ready to have a series about her, but by the time we see her in season 6 and 7, she's a pretty developed character. The core scoobies have always been set for spinoffs too, just depends on where their character is in growth at the time how it would go.
Willow has the magic and computer angle, and Xander always has the odd one out thing. I guess in season 3 the characters are still building some background on themselves for later, not all are ready obviously, but they're getting to spinoff territory.