If its just a small area reskimmed in a heated room, you could probably get away with this but I'd personally leave it longer - two weeks. It does take time for the moisture to come out of the plaster and 3 days isn't anything like enough for a completely reskimmed room.
You are right about watering down the first coat - because the plaster soaks up the first coat of emulsion like a sponge.
I have had my living room, Kitchen and bathroom skimmed. I would personally wait a week after a few days it may look dry but it isnt that just the front. Whit emulsion is fairly cheap so i dont think you need to water it down and you will get away with 2 coats doing it that way.