This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
How can a man have mutually safe sex with three women with only two condoms?
How can three men have mutually safe sex with one woman with two condoms?
Use both condoms on the first woman. Take off the outer condom (turning it
inside-out in the process) and set it aside. Use the inner condom alone on
the second woman. Put the outer condom back on. Use it on the third woman.
First man uses both condoms. Take off the outer condom (do NOT reverse
it) and have second man use it. First man takes off the inner condom
(turning it inside-out). Third man puts on this condom, followed by
second man's condom.