I think a fairly honest chat with her's going to get you further.
"Look, you come over as really ?????? (something nice and none slutty) and I obviously think a lot of you but since you mentioned the 3-somes it's been on my mind a lot, mind if we talk about it a bit?"
and take it from there, it's not hard. If she's well adjusted she'll see the underlying problem that's caused the question and be quite quick to dissolve your fears/issues. If she gets a bit pissed/upset you have chance to steer it to a better point yourself. "I'm not saying it's hurt my opinion of you but it's a bit... too... liberal? Compared to what I'm used to in a girl." explain your fears etc and expect a decent bit of comfort and re-assurance in return. I'd be kinda shocked if not to be honest.
You don't have to be on the verge of wanting to settle down with the girl to ask it, it's kinda cool even when it's casual to ask questions that relate to your own sexual well being "are you seeing anyone else" is a perfectly decent question regardless of casual or serious. As are follow ups about their other partners. Expect some to be a bit cagey and give the "well, if you don't like it" type response but... it's quite right. Your choice in the end what you stick it in, it's fair to be at least an informed choice tho or they aren't worth your time.
Also: very male reply - ask her if she'd be interested in picking up another chick with you and helping you do her