This is where a woman is penetrated by three men with the penises in different combinations in the anus or vagina eg 2 in the vagina and 1 in the anus. Hence, quadrouple penetration would be relatively simple by stationing another penis in front of the woman's mouth.
The term is often confused with an "airtight seal" which is where all three orifices are filled with a penis.
Many are skeptical about this as not many women are able to accomodate such a volume plus the restrictions in space make it difficult, plus many men aiming to penetrate a woman would not enjoy the concept of grinding their penis against another man's. However, it has been seen in a small volume of porn films and in the Annabel Chong sex marathon.
That girl was a real dirty loose whore last night, we managed triple penetration!