How old are they & how do they know each other?
When I was a student, I lived in several mixed households, and I never really had any attraction to them. Once you live with people in a platonic sense, you see them in a different light.
Think about it, he gets to see them all the time, (not just when they choose to see them).
1) He'll see them all without their glad-rags or make-up;
2) he'll be queuing for the bathroom in the morning (and have to go in after they[ve been);
3) he'll be in hell when it's "that time of the month" (and everyone has "synched" lol);
4) ]he'll see their dirty clothes and food utensils left around;
5) He may have even heard them "entertaining" other guys etc.
Also all of the above works both ways they'll have seen all of his bad habits, which will probably be off putting to them.
IMHO, they probably either do things all together or, they do their own things. The only time you'd have any concern is if he seemed particularly "pally" with one person (even then, it could be innocent)
Remember, guys and girls CAN be just friends.