So, it sounds like she is wanting poly and for you to be a throuple. While IMO there is nothing inherently wrong with wanting that it needs to be an enthusiastic yes for all members of the throuple for it to work and honestly even with an enthusiastic yes it often fails. I know because i have been poly and in a throuple. I am now no longer poly but in an open/swinging relationship. There are so many pitfalls to poly that I didn't even consider before trying it. That is all ok and some people make it work. But it sounds like this is a no for you and that is totally valid! If she doesn't respect your no she certainly wouldn't respect you in a throuple. If I were you I would have you and your boyfriend give her a hard no and ask for no contact. Then block if necessary.