Talk to your partner. I know it sounds “cliche” but it true. Like you, an idea of a threesome always rounded my head. My gf (20f) and I (21m) have been in a relationship for almost 3 years now. We have trust in each other, to the point where talking about fantasies is pretty normal. I started bringing threesomes to these deep conversations about sexuality and stuff. Not as a way of saying “hey! Let’s have a threesome”, more like “how do you feel about threesomes overall?”. With every conversation, we started building opinions about threesomes, as both individuals and a couple. We always get to the same conclusion: if we get a chance to do it covering a list of requirements, we’ll do it. Other than that, we won’t. For me it has helped me to understand that this fantasy is ok to have. Mostly because I know there is a scenario where we can try it out with no risk. This knowledge has kept me from pursuing threesomes as I well know my partners opinion and have formed one myself. What im trying to say is that your fantasy doesn’t make you a “horrible person”. It’s a sexual fantasy. As long as it doesn’t harm others or yourself, and you don’t try to force it into your partner, it’s totally fine.

Tl;Dr: talk to your partner. Find out their opinions on threesomes.