*Frosty will list all pros and cons for them... (all ignoring new stuff from FR/LG)
Salamence
pro: hugh movepool, with a great physical attack power and dragon dance, Salamence proves itself to be one of the most dangerous opponent (hmm... perhaps not most dangerous but still threatening)
con: x4 ice weakness, no defensive power... very inflexible as it can do nothing but sweep sweep and sweep more. Quite useless after it's paralyzed (then it's just free shots for ice beam, and that paralyze totally waste out the dragon dance speed boost)
Flygon
pro: levitate is always handy... quite average stats as Flygon's stats don't vary that much... it can somewhat special sweep with a little help from using earthquake as well. Highest defensive stats among the 3.
con: the only dragon type without dragon dance... earthquake and return along with HP is the only physical attacks it can ever learn that's decent. And it's not that flexible in special sweeping as well with limitations of crunch, flamethrower, hp, or solarbeam/sunny day. still x4 ice weakness which hurts
Exploud
pro: classical sleep talk, along with a great 2vs2 move hyper voice. the high hp makes it immune to seismic toss... Can learn extrasensory which is by far best psychic attacks (high HIGH PP). Like most other normal types, it have a good movepool in both the physical and special department. You can always have lots of fun risking swagger too =p *c*
con: weak weak defense... hopefully the high hp can help in that. Its ability is close to useless... plus there are much better normal types as well which is the primary reason why it's UU at the moment.*