kate1013
Potential
There are two possible answers to this.
1. It is a continuity error.
2. Buffy's yelled invitation, a general "Come in!", actually counted toward ALL the vampires in the vicinity. Therefore, the Three COULD have broken a window and gotten in, kicked down the door, come back later, etc. Perhaps they didn't realize the invitation had counted towards them as well, or perhaps the Master summoned them right then...and then, when they returned to the Master with news of Buffy escaping, Darla staked them. Which I believe was a stupid thing to do, had these vampires actually had access to Buffy's home, which of course the Master did not know about.
That's a very good theory.
And it's most likely what had happened, or at least what i'm going to believe what happened 🙂