As someone has said previously, one of the joys of a threesome is being able to take it at whatever pace you feel like. If you want to do her together, you can, if you want to be watched, you can, if you want to watch, you can.
I've had quite of few threesomes, first with a married couple for about a year, then a mate I was in the army with with several different women, it worked really well because the dynamics worked well:
- both of us enjoying the camaraderie of group sex
- both of us being exhibitionists and voyeurs
- him being dominant and me being pretty submissive, he liked stage-managing everything, telling us both to do stuff to each other while he fucked her or telling her to do stuff to him while I fucked her. We both really got off on spit-roasting and DP. He love watching his cum getting licked up and swallowed, whether it was the girl or me - basically the dominance/mild degradation really got him off (and me off too).
- both of us being very communicative during sex, both giving suggestions, making requests or giving compliments. We both loved it when girls completely lost it, my favourite position was with me on my back with her 69ing me, he'd fuck her doggy-style while I sucked her clit - girls would just go apeshit, absolutely lost in the pleasure of the moment.
I've had 3somes with just guys and the dynamics never worked out well, and the 1 FFM 3some I had was OK but I can't handle being the centre of attention the whole time (whereas with a MMF I can participate as much or as little as works).