How do you flirt with a couple/ escalate things into a threesome? (NSFW?)

I’m cautious by nature. I’m also openly bi. There have been times when I thought a threesome could happen, but I didn’t/ couldn’t go through with it, because I sensed that the attraction wasn’t equal all around or I sensed jealousy or reluctance from one of the partners. And just FYI, I’ve had plenty of one-on-one dating and relationships, and sometimes potential threesomes came up then, as well. Right now I just want to explore.

I want to make a good faith effort to make sure nobody’s feelings get hurt- if I get together with a couple, I don’t want to be part of the emotional life of their relationship. I want it to just be about the sex, I want it to be temporary/ occasional, and I want the relationship to be just as strong when I leave as it was before I got there. Like the campsite rule- leave it in better condition than the way you found it.

I know a couple now where it seems like it could work. I’m equally attracted to both of them, and they seem really solid. They’re gracious and affectionate toward each other. I’ve been friends with them for three years and I haven’t seen any weird emotional issues. In fact, they’re all-around awesome.

When we get together, they’re a little touchy-feely toward me. She’ll hold my hand or he’ll put an arm around both of us and stay like that for a while. Nothing happens that’s overtly flirtatious or over the line of regular friendliness, but they do touch me quite a bit. It’s affectionate, but I have no clear grounds to think that it’s attraction.

Of course, it’s very likely that they think of me only as a friend and that they aren’t interested at all. So what’s the best way to politely let them know:

1) I think they’re both hot and I want to have sex with them.
2) I don’t want to break them up, cheat with either of them, or stir things up between them. I’m only willing to go through with it if they’re both enthusiastically on board.
3) I want to be an occasional (or even just one-time) guest in their bed and that’s it. I don’t want to be a third party in their relationship relationship.
4) If they’re not interested, it’s cool.

I’ve done a lot of soul-searching about it and I can honestly say that all those statements are true. I’m single, I’m having fun, and I’m a free agent right now. Feelings can change and intensify after sex, but emotionally, I think we’re all in a pretty levelheaded, lighthearted place right now.

What’s the etiquette for graciously letting them know that I’m interested, and maybe someday being a guest in their bed? If it matters, I’m a girl. She’s bi and he’s straight. We’re all in our mid 20s.