I have no way to judge how she would behave in a healthy relationship because we've never seen her in one. I just know that, by her own admission, she likes trying new things sexually.
I think a threesome is well within her wheelhouse... under the right circumstances. Obviously a boyfriend having sex with another girl WITHOUT her permission is a non-starter. So why might she give permission? Maybe she wants to do something special for him. Maybe she wants a new experience. Hell, maybe she wants to sleep with a girl herself and figures this is an acceptable way to do it. I'm definitely not saying it's certain, just that I could see it happening based on what we know about Sage now.
Besides, if there's one thing we've learned about BaDIK, it's not to bet against girls being remarkably open to sexual experimentation with the MC!