Tony,
Pen size will be fine for those 3 species in pairs. Wood ducks are the flightiest of ALL migratory species, they will tolerate you in there, but move slowly, do what you have to do, and get out. Watch them, let them do all the moving, do not crowd them in a corner or anything and they'll just move away from you. Mandarins are the next flightiest of them so same thing. You'll never be able to get them to let you touch them or anything like that, but they will be just fine in the pen as long as you "ACT RIGHT" while you are in there. If they feel you are trying to catch them or crowd them, yes they will fly. BUT, you'll ne surprised at how well they will know their surroundings after a month or so in the pen.
Mine were all in 1/4 acre pens and they'd fly 60 miles an hour to the end, bank at the last second, miss the wire and do it again. They know where the ends are. So just move slow, be easy with them, and they'll all be fine.
A lot of cover in the pen will go a long way to calming them down, bushes, tree tops, anything they can get into and feel hidden. an open barren pen with no cover equals jumpy scared birds and none will evern breed for you.
Yes mandarins and wood ducks are about the easiest to keep, little harder on brooding, but good over all beginer ducks. The hoddeds will get along fine with them, they arent aggressive at all, they are a little harder to keep, will require you giving them floating catfish food or small pettet dog food for a protein boost to mimick their fish diet.
The most agressive ones you will have are the mandarins. They can be a bit grumpy in breeding season, but never cause any real damage, will just run the smaller ducks away from their area of the pen..
good luck with them, and by the way, all 3 are cavity nesters so you will need 3-5 nesting boxes for them.