True! It sounds like the LW isn't sure about that. This stands out to me:

I have never experienced something like this.

I'd probably recommend that she read up on polyamory (from reputable sources), keep checking in with herself, and see whether she gets more or less comfortable as time goes by. If she gets more comfortable, great! If she gets less comfortable or hits a hard limit, then alas incompatibility and gentle farewells.