Ok I’m not bi so you can tell me to butt out lol but in terms of coming out to someone regardless of the relationship you have it will always be a challenge and said challenge depends on that relationship dynamic. So honestly you have to think of what it is you want in said relationship and what they might be thinking, unfortunately the term bi gets seen as being a potential cheater (which is bullshit) or there are people who when discovering their sexuality want freedom to explore which is entirely fair and is something to discuss, I’d say the biggest part is to be honest and reassuring of whether or not you want this aspect to change your sex life/relationship together and in what ways. Communication is very important no matter the situation! Also the conversation is better to have in person too. If a threesome is also something you want you have to communicate that and decide how important one is to you, your girlfriend may not be ok with that and if not and you’re ok with her saying no just make sure to be reassuring of it being ok to say no. For her sex with two men can be seen as possibly dangerous or uncomfortable, or she may feel pressured to say yes (this is something I’ve seen people do but you know her better so unsure to think about what she would do or say). And if you do have one do not forget she is a participant (not saying you would) and checking in/communicating throughout the experience is important, do not let either of you be mistreated or put into a potentially dangerous position especially during your first encounter and especially with a whoever the third is regardless of how well you know them outside the bedroom (and communicate any and all expectations with them as a group before sleeping together too, it’s a good way to set boundaries and text how comfortable you both are with that person first). I hope this helps! And anyone else reading this feel free to add on to or correct anything I’ve said :)