It's an interesting story so far and I find the premise new and different (it's also the closest I've seen anyone come to crossing Harry Potter with Karl Edward Wagner's "Kane", though yours doesn't have quite the menace and background of his Kane. I'm looking foward to seeing who the third witch is, I rather suspect she might be Slytherin for only in the witch's bathroom there, with the tap running, could a witch indulge in tears, anywhere else 'twould a sign of weakness that would be exploited by her Housemates.
My only criticism is that you badly need a beta, there are times the spelling and homonym problems are rather distracting.
Author's response
your close on the bathroom bit. but still a little to inside the box with your thinking. and i don't think anyone ever found out what house she was in