It is probably a condition known as Scaley-face. It is actually cause by a minute mite that burrows in the cere. If you check the feet you may see a bit of white there too. This is easily treated with a paint-on solution available from most pet shops.
It is best to do all 3 budgies because scaley face is contagious in that the mites will travel to the nearest host. They may also already have the mite although it may take a while for it to show, so treat them all. It is real easy to do.