GaTrout,
After 3 years, you should have a seal...
Pond water loss can be from leaks, from tree uptake and respiration, wind and sun evaporation. First determine the normal evaporation rate for your immediate area. One way is to set a large dark trash can beside your pond and fill it with pond water and compare the loss in it to your pond.
If you feel it is leakage, call in the experts to determine if the leak can be stopped without a lot of cash outlay. For example, loose gravel and sand bottoms would require a lot of money whereas a clay bottom may require a relative small amount of benonite.
Good luck,