In terms of the pen, anything that supports their natural behaviour is good. Areas to dustbathe, natural cover, nesting areas, sunny spots, shady spots, plenty of vegetation and resources to forage, open spaces for the peacocks to display, areas for them to get away from each other, plenty of different roosts etc. Multiple feeding and drinking stations would also be important with multiple birds. You can get ideas in the stickies section or post some photos of your current set up if you want advice on what to improve or add.
You say you have 3 peacocks. How many peahens do you have? Or do you mean you have 3 peafowl in total? That size area you have is good for 3 peafowl but not big enough for 3 mature peacocks and enough hens for all of them.