回复：华尔街 Interview Questions, 18+ please

Try this way,

Condom 1 as (C1) and Condom 2 as (C2), Man A, B, C,

A uses double protection, meaning C1 on top of C2 then..., (after that, C1 outside is clean)..
B uses C2,
C, turns C1 inside out, then put C2 on the top...

3 cheap guys are all safe..