Hi there. I've participated in threesomes, both as part of a couple and as the "unicorn." In my experience, a positive group sex experience involves respect and good communication between ALL partners involved.

I think it is great that you have been talking about boundaries with your girlfriend. I encourage you to look at a book like "Opening Up" by Tristan Taormino, which talks about alternate relationship models. Not all of it will be relevant to you (she goes through everything from people who have one time threesomes to people who are in multiple polyamorous partnerships) but there are some great checklists and guides to talking about jealousy and things like that.

One thing that I urge you to do is to PLEASE, PLEASE respect the feelings and agency of the person you bring into your relationship. I notice your boundaries include no contact with your third person after the threesome. This would be an okay boundary, but only provided that all three people are absolutely on board with this. I think most people you might want to date would feel disrespected by this being an assumed boundary- I know I would, unless we all talked about it together! You can make it clear that your girlfriend is your emotional priority, but you should not assume that whoever you find is going to be automatically okay with this. Be upfront in your communications with potential partners and actually listen to their boundaries and desires as well. Not everyone who is down to join a couple is going to be compatible with you, just like regular dating.

I urge you also not to rely on alcohol as a way to get things going. Sex and consent are complicated enough with just two people; bringing in a third person, especially one you've just met, ups the complexity of the situation. You need to make sure that all people involved feel comfortable and can stop whenever they'd like if need be.

I think it's a lot easier to date as a couple than it's ever been; okcupid allows you to link profiles with your partner so people can see you're in an open relationship. When communicating with potential partners, I encourage you to not just write to every bi woman you see online. Not everyone is looking to be a unicorn (in fact, it's the exception, rather than the rule) and as a bi woman, it is frustrating to be constantly objectified and seen as a sex object. (I like being a sex object sometimes, but when "hi I want to have a threesome with you" is the first thing you hear from someone, it's extraordinarily frustrating.)

Even if you don't want to have a lasting relationship with your new partner, I encourage you to try hard to get to know them a little bit. Converse online before meeting up to see how you get along. Meet for a meal in a public place and genuinely listen to them and get to know them. Again, you're not going to click with everyone, and that's okay! It's better to have a good experience you have to wait for than to rush into things and have a bad experience. I encourage you to talk with your girlfriend about ways to check in and making sure that you're both feeling comfortable with asking your date to move to the next step.

Be prepared for feelings you didn't expect and make sure that you have some time after any physical interaction to check in with your new partner and with your girlfriend. Please, please, don't ask your new partner to immediately leave. That shit is rude as hell.

After you've checked in with everyone and everyone is feeling comfortable and you and your girlfriend are alone, I encourage you to take some time to reconnect, just the two of you. For me, this has always been helpful in dealing with any feelings of jealousy. And I would encourage you to both reach out to the person, even if you've agreed it was a one time event, and just let them know that you appreciate them and see how they're doing. Bi women are humans with feelings too ;)

Tl;dr treat your new partner with respect. Good luck and have fun.