I've been doing this for a while, getting threesomes with my girlfriend.

Tinder and Bumble are horrible for this - I did have some success in online dating. But the easiest way to do it is to go out clubbing together, and game girls together. You'll be surprised by how much easier it is compared with when you do this alone or with a guy wing because you've already got preselection, and thus won't be needy, desperate, or creepy anymore by default. Although you may have to introduce your girl to pickup to do it successfully, it's so much worth it if you want to have regular threesomes. In any case, if she becomes your wing and doesn't act weird all of a sudden - then going to clubs and bars is the best way to get consistent threesomes with her.

Just go there to have fun, and check the girls out together, and ask her which one she thinks looks hot and would be fun to hang out with. After she tells you, go up to that girl, and say "Hey, my girlfriend thinks you're hot - come meet her!". Then just taker her by the hand and introduce her to your gf. Start shooting the shit as normal, vibing, having fun, and using your regular shtick and some limited physicality to ramp up attraction etc, without making it look super obvious because the attention is now focused on your gf interacting with the new girl - not on you. Then, if you see you're all vibing great together, just go have fun all 3 - dance, drink, do whatever you do to genuinely have fun. After she sees that you and your girlfriend are awesome people - just invite her in, again using your gf as an excuse, by saying something like "Hey, my girlfriend really enjoys how fun you are blah blah, let's go party at our place". That works most of the time because it's suddenly not some guy asking her to come over like she's used to most of the time - but it's completely nonchalant and "harmless" since it's some guy's gf who invites her for an afterparty. In my experience, this works wonders most of the time. (nothing works 100% and i've tried tons of different things since i've been doing this for a decade lol).

So once you're all home together, then make the setting as cozy, comfortable, chill as possible (lighting, mood, music), and then both you and your gf should start building up tons of comfort and rapport with the new girl. After that, comes the sexual tension. And this one's easy in such a case as again it's not what the girl is used to - while vibing or playing some silly games or whatever you do, ask if she's ever kissed another girl - if yes then great and your gf should take over. if not, then say it's the perfect opportunity to experiment and even joke about how it's all going to be your little secret. From here, if your girlfriend is awesome, she can kiss her and go into a huge makeout and it's pretty easy to go from there - but your gf really has to be active in this, and not remain passive because it's so much harder if it's your job suddenly to keep them both on their toes. That's the hardest part tbh, to make your gf be an active participant and not just a "spectator of the show" where you have to do everything yourself.

Another great way to initiate the threesome when you're all back at your place is through massages, but it would take too long to explain here. Again, tried tons of shit, and this is what works CONSISTENTLY.

Getting threesomes with your gf is awesome because it has the added benefit of showing your gf that you're THE MAN since she'll attribute you both getting a new girl to YOU, which is alpha as fuck. (if you don't subscribe to the alpha/beta mentality and explanations, let's call it masculine as fuck

I'm actually thinking of doing some infield videos and maybe a course of how to get threesomes with your gf since i'm already regularly doing this.