3" in 2 months. I wouldnt lose any sleep over it. Open the pool and try to measure a loss in 1 week.

You may be surprised to know that your plastic winter cover will actually allow water through on a micro basis given enough time.

If you have rain water lying on your cover, it will exert pressure on your cover and you actually may have a little water seep up through and evaporate.

I'd turn on the hose for a few hours to gain the lost water and start swimming.