Hey, lurker here. I just ran across this awesome pts narusasusaku fic by Sunfreak, and I wondered if you'd already read it. If not, I highly recommend it. Actually, I think you might find it interesting just because it has a take on how the three of them would relate to eachother in bed which is very different from yours, and yet is quite plausible and in-character. Check it out if you're interested.
http://community.livejournal.com/white_knuckle/32713.html#cutid1