Make sure your equipment is in perfect working order
before you need it. There's nothing worse (well there is, but it's annoying) than having that precious weather window while your machinery stands idle.
Get in a stock of shear bolts for the baler.
Use jute twine not polyprop, if the string cutter on your baler is at all dodgy.
Get someone to show you how to tell when your hay is absolutely crispy dry.
Don't bale damp or underdried hay.
As others have said, how long it will take depends largely on where you are and the grass you are dealing with, but things such as overnight mist or dew which takes a while to burn off and a lack of breeze will slow you down.
As Sallyintnorth has said, up here we need 5 days to make and bale good hay, longer if it gets rained on
modified: I've just seen that you are near us, so I very much doubt you will make hay in three days, especially now we are into August. We are hoping to go for it on Friday. We know it will be rained on Sat, but then there are a few good days forecast, although not much of a breeze