Did all the eggs look the same? Same size ? same color? My guess would be that the same duck layed all three eggs( I think you described where you found them). I would assume that the duck that layed the eggs was a female mallard and that the duck that fathered 2 of those eggs was a male mallard. Now the third one I think is probably a mallard cross. Probably crossed with some other mallard derived breed. If it was crossed with a scovy it would not have hatched at the same time as the other 2. Of course this is just a theory. They are very cute!