Really? I was going to say the opposite. You have two hot people who you like, calling you and telling you you're hot and they like you. So... enjoy!
You could also fly/drive over there a couple times a year, or whatever frequency is feasible, and go have some more threesomes. And then just sort of see where it goes. It can still develop organically with you far away, but it will also develop slower, so don't try to turn the relationship into something it's not yet. Just enjoy your phone time and maybe go see them sometimes.