Nature would always depends on moveset and your playstyle. Particulary i think best nature for Hitmonchan is Adamant. And 76 stat speed is not poor, actually it is really good for this wallbreaker role, poor speed is 50 or less stat base but also on this cases you have to study if some evs invest would give you some adventages.
So my answer is Adamant 252 Atack and 252 Speed for Hitmonchan.
About Hydreigon and Haxorus also depends on their sets so this are my answers for them:
Hydreigon: Choise Scarf Modest, Choise Spects Timid, Taunt Timid.
Haxourus: Choise Band Adamant, Swords Dance Jolly, Dragon Dance Jolly.