Two of my friends have been dating for a little while. I spend a lot of time with them and I know a lot about their relationship, to the point that we often joke about the three of us dating. How could I encourage/introduce this idea without being too bold about it?

Why are you worried about being “too bold”? Is there a specific concern you have, a reaction you definitely don’t want? In my experience, it’s nearly impossible to have a conversation like this without being bold.

You gotta just bring it up with honesty and openness. “Hey, you know how we joke sometimes about the three of us dating? Do you think that’s actually something you’d be interested in trying?” If they go “oh my god, no, I’m sorry, that really is just a joke!” then you have your answer - but if they are also interested, you can have a conversation about every person’s fears, desires, etc. Good luck!