These positions involve anal penetration: Doggy style penetration maximizes the depth of penetration, but can pose the risk of pushing against the sigmoid colon. If the receiving partner is male, this increases the chances of stimulating the prostate. The penetrating partner controls the thrusting rhythm.
I would refer to a fisting - also not an easiest practice for newbies, but very pleasant if it performed in a right way and with a right partner. And that is a very popular activity, especially in BDSM community. Fisting, penetrating a vagina with a fist, is also needs good preparation and arousal of a woman. Probably, fist may take even more place than 2 penises.
D E The above illustrations show the area between the labia minora. From top to bottom can be clearly seen the clitoris, urethral opening, and vaginal opening. A, B, and C show vaginal openings with a normal hymen, a membrane that partially covers the opening. The hymen is the traditional "symbol" of virginity, although being a very thin membrane, it can be torn by vigorous exercise or the insertion of a tampon. Illustration D shows an imperforate hymen that completely closes the vagina; this rare condition requires surgical intervention to provide for a normal flow of blood once menstruation begins.
If you have, you are in for an amazing experience, but you still may have questions, especially about double penetration. First of all double penetration is not a requirement of an MFM threesome. The men can just take turns or you could just do oral along with vaginal or anal sex.