hmm I think with my friends we regularly do activities where everyone brings their partner, and also stuff where it's the "original group". But, we've all know each other for a very long time, so maybe that makes things different. I also think it can be fun to hang out with a couple when you like both of them, but it's definitely weirder when you don't know the so.

Could you occasionally suggest stuff to do where there is a set number of people, like "I have three tickets to this show"? I guess they might just say no tho. Honestly it might turn out that as people find relationships your friends have less time and attention for you, and if you're lonely you'll have to meet some new people. Getting old sucks.

Hey man, sorry that you're feeling this way in your friend circle, I'm kinda in a similar situation myself so I can empathize

To answer your actual question, I think something short and simple would probably be best. Start with a greeting, what you want, and why. An example would be something like, "Hey [name], I don't want to be friends anymore. I loved hanging out with you but I just don't want to feel like a third wheel". Add more detail if you want, maybe a sorry at the end. Use it more as a starting point. In the end it's up to you, that's just how would do it