How can I add a second light switch to a 3 gang box with 3 switches in it?
Don't bother with a traditional wiring solution. 

Get a smart switch that's designed to be "3-way'ed" with a remote switch.   Preferably a remote that communicates with the main switch wirelessly (via Z-Wave etc.) And most preferably, a remote that is powered by batteries.  That way you can just stick it to the wall, and not have to tear up walls or posts and refinish in order to run wiring to the other switch.  If there's already wiring at the other switch location and you can get hot and neutral, you could use an AC powered (but still wireless) remote switch.   

In this solution, the only additional wire will be a pigtail from the neutral already in the box, to the smart switch. 



If you really want to go traditional, you will need to fish wires and refinish, as needed, to run a cable from the current switch location to the new one.  You can use the usual 3-way switch diagrams, with a twist: you must also carry a neutral wire to the new location. That means you must use 14/4 or 14/2/2 wire instead of the 14/3 all those diagrams call for.  (if the circuit is 20 amps, this must be 12/ wire.)  Also, you may also need to change the circuit breaker on that circuit to GFCI.  You will have 4 additional wires coming into the box (from the 14/4). 

You won't need to do that for changing to a smart switch; that counts as "replacing a switch" which is considered a trivial repair.  I've never heard of a municipality requiring a permit to change a switch. 



The neutral wire is required as of 2011 by NEC 404.2(C).  An "Exception #2" was added for indoor 3-way circuits, but this is outdoors. 

A GFCI is required by NEC 210.8(A)(3) if receptacles (outlets) are on the circuit.  That is likely, since other parts of Code require at least one outdoor outlet.