First mechanic would be to find a 4th official.
No you don't give any signal that a fair catch was called. Once you see a fair catch signal given while the ball is in the air you know that the player and only the player who signaled is provided protection from being contacted after catching the ball. All other players on that team are allowed to catch the ball with no protection unless they also signal. The ball becomes dead as soon as anyone possesses it. It is the responsibility of the kicking team players to know if there was a signal.
The NF mechanics for 3-man are at the following site
http://www.nfhs.org/PDF/Football/ThreeOfficials.pdf in case you didn't know.