all 3 of these classes are competitive dps wise in upper end instances (toc 25 man, Uld 25), if anything the mage tops the other 2 in most fights, but that is from my experiences in my guild, I myself play a survival hunter and on most fights I am 1st or 2nd in dps, on some fights the top dps in guild may only differ .3% to .5% dps.
I can't personally speak for the rotation of mages at high end as my current mage is only 74, but hunters need more micro management then ever before (and I've played a hunter at high end since vanilla), warlocks current destructo rotation is not a face roll either but once you get when are where to use conflag and chaos bolt it's pretty easy.
Hunters role in expansions has change a bit over the years e.g. Hunters in vanilla were pretty much required to separate packs, now not so much, they do however now bring mana regen buff and attack power buff depending on talents and pet though.
Warlocks bring the same utility they have brought in previous expansions,
healthstones (health pots basically),
soulstones, (enables a player to resurrect once every 30mins),
summoning for players in group, ability to
banish certain targets e.g. elementals and various small buffs from pets like extra health when imp is out and extra mana/intel when felhunter is out, almost forgot one of the debuff provided by locks (
curse of the elements) magical damage increase to target which is pretty substantial.
Mages bring a
mana/intellect buff, ability to
sheep/polymorph (crowd control a target for up to30 seconds or so), ability to remove curses and health and mana food (
conjured refreshment) for out of combat regen.
Some people have mentioned that mages and warlocks are kind of squish, in a pve sense that is true in a pvp sense they can indeed be some of the hardest to take down, frost mages have crazy survivability with their ice barrier couple with ice block,
mana shield,
frost ward and
fire ward and their pet when used probably can keep a melee character crowd controlled and add damage by frosting the target.
Warlocks once they have some resilience can be hard to take down when you couple
soul link (ability to split damage taken with pet), sacrifice and health regen from spells like
drain life,
haunt and talent based health regen.
Hunters suvivablity is governed by their abilty to avoid being hit, part of this is kiting part of it is the use of pet e.g.
pin is a crab still that holds target in place for 4seconds, the use of traps e.g.
frost trap slows enemies in area will be slowed by 50%, their other option is to use
disengage a fairly recent change to an old skill, it used to reduce agro by trying to take you out of combat it now makes you leap back which is great for keeping your distance.
Feign death can be very helpful in pve and pvp.
Like I tell any new player, try the classes out and see what you like, good to see you have done some basic research and are asking for more info tho.
I myself play a hunter at moment I always seem to come back to it, same as my lock, then again though I have an 80 of pretty much everything minus warrior (73) and mage (74), so I do chop and change a bit.