I was single when my friend, Eric, started chatting with me more regularly online. It was really nice catching up with him, and after about a week of this he said, roughly, "You know that Gia [his wife] and I have been involved with other women before. Well, after Jen [their ex] we decided to close our relationship for a while. Now we're feeling ready to open back up and you're on the top of our list for a threesome. Let me know if you're interested!" EXTREMELY straightforward, and I appreciated that.
I thought about it for a few days and then decided to go for it. For a few months I was a friend with benefits with them, and then Gia and I started hanging out more and really connecting and we decided to call ourselves "girlfriends". Now, three years later, we have a really close emotional relationship, and a really hot physical one that still occasionally includes Eric, and this is all in the context of having no "formal" commitment to each other, no formal schedule for seeing each other, rather infrequent dates, but a great deal of comfort and camaraderie and mutual support.
Obviously your situation is different in a couple of ways, but I thought it might be helpful to see one example of how things can start and then progress. Long live the straightforward approach!
Best of luck.