Three penises or objects that exemplify penises, such as dildos, in one anus.
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.
Cindy tried triple anal with her boyfriend and two dildos
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.