What are you painting with?
Brush & Roll? Hm... after 3 coats it should be pretty well covered...
Spray Paint? <shivers> I can never get spray paint to look right on large surfaces.
Spray Rig? Oh goodness I hope not... If you can't get it right by the 3rd coat, you've got problems
I'm going to assume you're using spray paint because thats the one most people try & fail at. I personally have never gotten a good even finish with spray paint, but most people around here say the following:
Only apply in light coats.
Sand smooth after each layer (except the final layer).
Take your time & make sure it's completely dry before applying the next coat.
Good luck!