With gamebirds, bigger is better....build the biggest enclosure you can afford to or have the space available. 1 to 3 birds can do well in a 125 to 150 sq.ft. enclosure....that would be a 'minimum' for 3 birds, possibly 5-6 birds if well landscaped.
A balanced and complete gamebird feed of 24% protein should be offered to your birds. Clean, fresh water and plenty of shade if in a hot climate. In general, the True pheasants, ie; Phasianus colchicus are very tolerant of cold weather, as long as they have protection from the elements and have a 'draft free' zone in their enclosure.
The enclosure should be built to protect your birds from predators, either 1/2" HC or 1/2" x 1" welded wire fencing works pretty well to keep most predators out of the enclosure. An apron should also be installed completely around the entire enclosure, to keep predators from digging under the perimeter of the enclosure. 2 feet is generally a good distance from the enclosure's perimeter walls/wire.
I bury my aprons about 8" deep and then cover the wire with concrete but soil will work, too! if you don't want the added expense.
Also, in general, pheasants can not be 'free ranged', once they escape or inadvertently get out, they will be gone or predators will have them in quick chilly minute.
HTH