I've got some experience with how to tell who's laying. Grab the duck, wrap it in a towel, put her on her back and put food coloring around her vent. Obviously different colors! It helps to have 2 people for this because the vent isn't all that easy to find, at least for me it wasn't. You've got 3 ducks so you only really have to do this to 2 of them and in the morning you'll be able to tell who's doing what!
I did my Eva's vent blue when she was having her issues back the beginning of February and I can still see it on her butt feathers but it's not on the eggs still.