please make me understand what should be done in such a time

So hello, I am a 21 year old boy and my girlfriend is also 21 years old. It's been almost 1.5 years since our relationship started. I just want to try threesome once, so I told this to my girlfriend as well, but I am unable to understand that sometimes she says yes and sometimes no, what she wants, she is not interested in sex and I have told her many times about sex life but she never pays any attention to me and she just wants to take the relationship forward without sex, and I really feel like having sex and I don't want to deceive her either, what is your opinion in this, please make me understand what should be done in such a time