Outdoor three way switch and outlet on light pole GFI
One way to add GFCI protection for the outdoor lights and receptacle would be to use a GFCI breaker for the circuit.  This will protect everything on the circuit, if there's anything else on it.  

That may be a bonus, or may be a problem if you don't want GFCI protection on some of what's on that circuit.  For example, if the sump pump in your basement is on that circuit, a nuisance GFCI trip could prevent the pump from working and you wind up with a flooded basement.  

Another way would be to add GFCI protection anywhere between the panel and where the cable leaves the house.  One way to do that would be to add a GFCI receptacle anywhere between the panel and the inside three way switch, maybe somewhere in the basement where the cable feeding the inside 3-way switch is accessible.  The receptacles have LINE and LOAD side connections;  you'd connect the LINE side to the power feed coming in, and the LOAD side to the outside lights etc. downstream.  Depending on the routing of the circuits wiring, other loads, etc., other loads may be affected as with a breaker.  If you install the GFCI receptacle right where the circuit comes out of the panel, it will affect all loads as would a GFCI breaker.  If you install it where the cable to the pole exits the basement, it would only protect what's outside.  

Another way would be to add a gang to the switch box and put a dead front GFCI in the switch box.  I like this method because people can see the dead front right next to the switch if it trips and not have to think too hard.