Most of it is based around how you build your character, what you spend your gold on, what guild skills you've bought, what astrals you have, and what gear you have.
Yes a 30 Mage can beat a 39 Knight if they have added balens and got the good jewelry, but other than that the 35+ knight should win due to you can kill their troops fast.
My 36 Knight killed a 39 Archer in Battleground today both started at full hp. Granted my knight is damage built.
Archers get 5 Rage per crit.
Knights get 10 Rage per Action no matter what it is.
Mages gain 2 Rage per time they damage an opponent.
Rage Build Abilities
Archer Arrow Strike: Gain 5 rage base +5 more if crit.
Knight Slasher: Gain 15 Rage.(5 Base +10 Passive)
Mage Lightning Bolt: Gain 8 Rage(6 Base +2 Passive)
Mages have no abilities that cost rage that they will break even on in a Duel/Arena/Battleground. Cheapest one is Rain of Fire 16 Rage, most they will gain back in a 3v3 situation is 12 rage.
Archers break even on Multishot and Lunatic Fire if they have really high crit. Rest of their abilities will give them most 5 rage.
Knights break even with Ultimate Slasher. Rest will always give back 10 rage base.
Area of Effect Abilities
Knights have an AoE it's called Whirlwind you can take it after Delphic Destroyer. It hits 3 Times +Additional damage proc. Costs 50 Rage 30 seconds Cooldown.
Archer's AoE costs 45 Rage hits enemies 3 times each, and has increased crit chance. 20 Second Cooldown.
Mage's AoE only hits targets 1-2 times costs less rage because of rage limitations of mages.
Knight: 10 Rage per action, +15% PAtk, +20% PDef, +20% HP, +20% Healing Received, Chance to restore 4% HP every block, Damage Reduction 7%, Crit Receive Rate reduced by 5%
Archer: 5 Rage per crit, +15% PAtk, +20% Crit, +20% HP, After a Crit boost Damage for by 2% stacks 5 times for 5 turns.
Mage: 2 Rage per time dealt damage to opponent, +15% MAtk, +8% Healing ability, +20% MDef, 8% chance to reduce rage of next attack by 20.
Each of the classes are pretty balanced. It all depends on your play style for what you'll be good at with it and the skills you use.
Talents which will unlock at 50 in a near update can reduce the rage and cooldown of abilities as well as grant additional bonuses.