I know this post is like a month old, but I saw it and wanted to share.
About a year ago, I had a threesome with a couple. It was pretty fucking great.
One of them -- I'll call him Jason -- contacted me on Grindr and we got to talking. Jason thought I was cute, I thought he and his boyfriend -- I'll call him Tyler -- were cute. Jason was a top, Tyler was a bottom. I told him I would want to top. The whole thing started to sound more and more enticing, especially because they both seemed comfortable with it. We decided to meet for drinks and take it from there. It went well; they were both cute, smart, and decently friendly. I think that step of the process was pretty important. It allowed everyone to make sure that the real-life chemistry (not just the Grindr flirtation) was right, and it also allowed for me me to be as sure as I was gonna be that these two dudes were not murderers or highwaymen (they were friendly, but not desperate; personable, but not sociopath-level charming). We picked up some condoms and headed to Jason's place.
In Jason's bedroom, I remarked on Jason's book collection just to have something to say, just to cut any tension. Pretty soon, though, we were all standing close to each other, taking glances... then we got down to business. It was like a wave of sensation. One was kissing my lips, another my neck, then one of them was down on my cock, then I found myself sucking Jason's cock, while Tyler was playing with my ass...It continued on like that for a while -- just this foreplay smorgasbord, lips and cocks and hands and chests and balls. The best sex I've ever had was one-on-one with someone I loved, but this was the most sensory sex I've ever had. There were just so many ways to touch and be touched. It was like sex in technicolor.
After the foreplay, I fucked Tyler doggy-style while he sucked Jason's cock, then Jason and I switched positions, and then I fucked Tyler missionary until we came (almost simultaneously). We got started up again, and the second round was a lot like the first, but just with a little less foreplay, and with one other cool component: at one point, I laid down against Tyler as Jason fucked him missionary. Their connection was so intense. On the one hand, the experience was voyeuristic, but at the same time I felt the heat and weight of it happening up close. It was great. We all came again. We parted ways with big fucking grins.
To answer a couple of your questions a bit more directly: I agree with the other posters that having ground rules is a good idea. You can bring up the idea totally casually and see how they respond. It's also good to get a sense of how open (or not) everyone is to anal. Who would top or bottom for whom, how important (or not) is anal? Some guys (guys who I don't like) feel really entitled to anal, even if you weren't expecting it, which is annoying. Make sure you're all on the same page on that front.
