I don't think you can write a MMF so that everyone has a chance to be the center of attention.
Ironically, in my situation, many of our threesomes have 2 people in the middle and 1 person on the outside. No kidding! Both of my long-term partners like threesomes where they get lots of face to face intimacy, and I have a more peripheral role. They lie down as if they were in missionary position so they can kiss. From there, if I enter the bottom one, then I am in a missionary position with her, and if I pull out and enter my other partner, I am in doggy-style with her. I can push in deep, bend over in the doggy position, and that lets me have a 3-way kiss with each one. In these threesomes, the women are at the center of each other's attention and of mine - and I am pleasuring them. I write out these threesomes as FFM since the two women come first.
Then there are threesomes where I lay back and both women are on top - one riding my cock and the other riding my face. Then they switch. Who says a man can't service 2 masters? That's another FFm threesome.
Sometimes I turn around the situation so I am the center of attention and they pleasure me. But I if I wrote out that 3-some as FMF it makes it look as if I am with 2 hetero women that don't interact sexually. So I write it out as Mff to indicate I'm the top, and the women are bottoms.