It is physically possible to achieve naturally ( without the use of toys ) but has rarely been attempted in pornography, certainly not in the mainstream.
However, triple anal penetration with the use of two penises and one dildo has been done, notably by female porn star Sandra Romain, twice.
Samantha had Frank and Derek sexually penetrate her anus at the same time, and also added a dildo into the mix
Somehow, some way, Jessica got three men to fit their erect penises into her anus all at once.
Has been performed once successfully on mainstream pornography by Sandra Romain in the hardcore flick "Tournante" aka Tournament of Sex. The camera angle is somewhat off, however for the brief period of time, it is one very hot position. mmm 3 cocks in 1 ass!