A form of gay sex involving three men. Man A inserts his erect penis up Man B's urethra. Next, Man C inserts his erect penis up Man B's anus and into the urethra of Man A.
It's a urethra within a urethra within a urethra.
Gosh darn it, Edward sure loves it when him and his two friends engage in a rousing bout of triple urethral.