3 hour wait for wave start...what to do?

That means you will have over 4.5 hours from when transition opens and your race.  Get to transition as soon as it opens, get your gear set up quick, then get out of there and go back to your hotel and relax.....you'll be WAY ahead of the folks hanging around in the heat.  Your wave will NOT go off early.

This is what I would be doing.

x3.  Plus use this time to your advantage by sitting quietly and visualizing your entire race. Visualize every detail starting with lining up for the swim start. Think about the sounds of the other athletes chattering around you, and the smell of the water. Imagine the countdown to the horn going off, and how you calmly begin swimming, sighting off the other swimmers, buoys, and landmarks. Think about your best quality swim form, what the right effort level feels like, and how the water feels sliding across you. Continue this visualization technique with your eyes closed and relaxed steady breathing for the full event all the way through the swim exit, T1, bike, T2, and the run. If possible, run through the entire race in your mind 2 or 3 times.