Taboos? Don't talk about sex, heh.
What are DR's and S's interests? Do they have any common interests? Even if they don't, you can always talk about one of the guys' interests, and then the other guy's interests. Trade off that way.
Things probably aren't as critical as they seem to be. It might be just as well to give DR and S some "verbal space" where they can just talk to each other and decide on their own subjects.
At the same time, there's always likely to be some awkwardness when anyone has just met and is just getting to know each other. Don't panic if there's some awkward silence. Talk about the economy and the weather if you get in a tight spot.
The trick here, I think, is to not try to "run" the conversation, but let it go where it will a little; give it some freedom. Hopefully this is just the first of many get-togethers for the three of you, so you have some time to "develop a comfortable dynamic."
If you're nervous, DR and S might both be nervous as well. So, you might all be wondering what to say and what not to say. Which is fine, as long as you don't drive yourselves crazy trying too hard. There might be some weirdness in the beginning, but that's okay.
If this were a movie, I could hand you all scripts and you could just memorize your parts. In a real-live situation, however, you'll all have to think on your feet. There's only so much planning you can do ahead of time.
Hah. If you've got any valium, now might be a good time to take some. If you guys drink, you might want to serve a little wine or something too. Whatever you can do to relax a little bit.
Note: This advice is all coming from a social introvert, so if it seems a little lame, well, grant me a handicap. My MFM "V" goes to a poly get-together about once a month, and I have the worst time relaxing and chatting with the other poly people there. It takes me a long time to get to know people, and I absolutely suck at remembering names. So there's one silver lining, at least all three of you know each other's names.
Seriously, I think things'll be okay; you'll survive this. Not that I can give you a guarantee about that, but my spider sense isn't tingling, so I think it's hopeful.
If I think of any other ideas, I'll let you know.