This is so going to stir things up, I'll probably hide under the desk a day or two after I've said it, but after many hours of writing myself into, out of and through it, looking at it from every possible angle I've come to the conclusion that, for me anyway, Jack/D/R is an OT2.5.
I can imagine (though not actually write) them having the most wonderful time as a threesome. And I find D/J absolutely beautiful and totally right. The one I can't get my head around is Jack and Rose without the Doctor. I have read some fantastic fic about it and what I'm about to say is absolutely not a comment on the quality of those works. But my reaction to the pairing is always detached - kind of, okay that works in the context of the story, but I personally don't ship them.
No, that's not quite accurate. I think Jack and Rose love each other to bits as friends and there's clearly sexual chemistry between them. Also, Jack's a character who tends to express himself sexually, sometimes to the exclusion of other forms of intimacy. For me (and it's a personal opinion which I don't automatically expect anybody to share if they don't want to) the issue is the nature of the D/R ship. And that comes down to the way I read the Doctor as a character.
I see him as a very vulnerable person who clings to Rose as a lifebelt, even more so post-Doomsday (assuming it happens). So much so that I think he'd be almost too demanding for her to handle long-term without some release. It probably says something about my own attitude to physical relationships that the release wouldn't be sex with another partner, for her, but for the Doctor to have a second partner .
For me, the centre of Rose and Jack's life isn't their love for one another, but their love for the Doctor. It's the deepest bond they share. And they offer him different, but complimentary, things. Rose is the nurturer, the one who brings him down to his emotional core. Jack has gone through so much of the same kind of pain as the Doctor and understands why he approaches things the way he does. Together, Rose and Jack lighten him up and give him friendship and a wonderful grounding. If either one of them, alone, was in a relationship with him, it wouldn't be as complete. We saw Ten/Rose exclusively in S2 - enough said. And although I imagine Jack/Ten having interludes of fantastic physical and emotional closeness from time to time, they both understand each other's defence mechanisms too well for that to become a permanent relationship without Rose's presence.
I don't imagine Rose having any problem with Jack and Ten having a physical relationship, quite the reverse in fact. She loves them both deeply and would rejoice at them getting something they so clearly needed. Particularly as she'll eventually age and die. But I think Ten would be too emotionally vulnerable to handle Rose sleeping with Jack. It's just the way I see him. He wouldn't admit it, he'd say it was fine with him, of course, but it wouldn't be, and therein lies the danger. Rose would know that, and even if Jack didn't agree I think he'd accept her decision.
The interesting thing is I've never read a single reading of the threesome that reflects my own. Maybe people are scared it sounds anti-gay or narrow-minded or something, but it's not. I'm fine with the slash and the OT3. It's just a matter of how I see the characters and it'd be interesting to see if anyone else understands or agrees. Of course, there may be people who feel angry with me for casting Rose in the sacrificing female role, because I'm sure that she'd enjoy a relationship with Jack enormously if she didn't feel it was hurting the Doctor too much. It's just, I think she would.