In theory, I think there are two different issues:
1. STD: if all participants trusts each other, as it comes to personal hygiene and health, then going bareback shouldn't be a problem. You will be exchanging body fluids any way, with or without a condom (sweat, saliva, pre-cum, etc).
2. Pregnancy: I guess its all participants interest and responsibility to decide how to keep this under control. Condoms is one way, diaphragm is another one, the pill, IUV, etc. Youn got plenty of methods (I'm not here to teach anyone anything).
Since I never done a 3sum (sad but true..) all the above said is based on common sense and not on experience.
My two-cents....
XRM