Not having all possible options immediately at your disposal is a pretty big no-no for RTS games as they're competitive by nature (even if played casually). Dawn of War 3 has a gate on Doctrines however and serious players are not happy about it. Experience points are fairly easy to get in this game so it seems it was made by the developers as a sense of progression or to restrict options to not overwhelm players. For those who want to spend less time grinding there are ways to make it faster like specific campaign missions or by having a friend throw units at you with high resources on. The best way of grinding experience however is with a custom map designed to give you the most experience with the least amount of risk in the fastest time.

The map currently used by the community is located in the Steam Workshop and is called 'Beat'em up (Elite Grinder 1v3)'. Go to the link and hit 'Subscribe' while being logged in on Steam to download and if you do so while the game itself is running you will need to restart it for the map to show. In order to start it you will need to go to 'Custom Online', pick your map and then use the settings 'High Resources, Fixed Positions, put it on Hard difficulty and use all Orks as opponents'. The mobs will come in a single area and be bunched up against a wall stopping them from getting to you. Simply farm those mobs and then when you're done destroy the Power Core that's nearby to end the match. Here are some extra tips I've found as well from trying it out:

• Go 'Custom Online'. No exp if solo
  
• Click 'Orks' and put on 'Hard' difficulty. You want easy to kill units like Sluggas. If you pick Space Marines they'll use Drop Pods to get to you
  
• Beware of Elites that have True damage or can cross the bridge like Gorgutz
  
• Just make an army of suppression teams behind you if Elites crossing the bridge is a problem
  
• Go forward a bit if the Orks aren't bunching up nicely for you
  
• Go back a bit if enemy Elites start using their abilities on you
  
• You might take the AI's spot if on 'Random' as opposed to 'Fixed Positions'
  
• There's a stack of Generators that get destroyed by the Turret at start and you get tons of elite points from it
  
• There's a Falcon at base. Load your melee elites into it and drop off past the wall
  
• Infantry elites will have to get healed at the Stronghold
  
• Consider using two infantry Elites at once and interchanging them between healing
  
• Vehicle elites can just have an army of builder units behind them
  
• You can alt-tab or go AFK if you have a ranged vehicle elite as their heavy armour means they take very little damage
  
• There's a ramp in-front of the Turret so it can't shoot you
  
• Go to 'Options' and set your master volume all the way to the bottom so it's muted and won't bother you while alt-tabbed
  
• You can also use this map to test units and doctrines if you would like
  
• Make sure to turn off doctrines and army abilities if not
  
• If there's a Steam disconnect or crash you will have to restart and you will get no experience points for it
  
• The time it takes will vary between elites. Some will be quick like Solaria and some will take way longer. For the quick ones it could take as little as under an hour and for others you should just try and get them up a level a day

There are 10 levels per Elite and a different reward per level. Some of the rewards are purely cosmetic and some are actually useful. The main thing you want is to get to level 8 as your Command doctrine will be equippable as a regular doctrine. Levels 9 and 10 are mainly cosmetic but the Hero's Mark can make it easier to see your Elite in battle. Anyways, here's the list of rewards:

Level 1: Your presence doctrine. You start with this
Level 2: Nothing
Level 3: Command doctrine
Level 4: Nothing
Level 5: Portrait
Level 6: Army Painter colour
Level 7: Nothing
Level 8: Command doctrines are equippable as regular doctrines
Level 9: Mastery Skin
Level 10: Hero's Mark