I am going to be playing a mini torny against some friends. We are planning to use between 2500 or 3000 points. There are 3 of them with the following army. Britons, empire and dwarfs. Any hint or tips on building a winning DoC army against them?

Considering that two of those are the mane artillery armies, I'd say you'll most likely need some pretty fast moving units, such as the flying guys and other quick models. I am not very accustomed to DoC yet, but I believe you'll want to reach them quickly (cannons in 8th ed have certainly turned scary).

What do you think about using Lore of Metal on a Herald of Tzeentch? Should be pretty helpful. Furies and Fiends would be nice also. Like Saintspirit said, getting to warmachines quickly should be beneficial.

Meh, 2 units of furies and a couple solo fiends and you should be set. Just remember when you destroy or break a unit EVERY unit in 6" of the unit has to take a panic check or it flees (Fails to fire next round in the case of Warmachines) With their BSB and General probably far away from them you will be surprised how many fail their panic check.

As for Dwarves and Brets... to be honest, they don't pose much of a threat to Demons. Crash into them and let your demons do what they do best. Just make sure you don't get hit by multiple lances without support ot you'll be in serious trouble.

Mind where you place your GD if you take one. (At these point values you should) and try to get them into combat fast so that they can't be targeted anymore by warmachines.

Only the new empire should give you any real problems. The Warrior priests are dnagerous and really boost the units (Detachment and regiment) rather easily so take them out fast. OTher than that remember the detachments are viable now so treat them you would any other regular unit of however many models. And watch those chocobo units (Monst cav) THey are awesome!

Good luck! Post Bat reps and pics!

-Tremendous

Well, lore of light is pretty awesome against all of those armies. Against Emprie and dwarves shooting it gives good protection and against bretonnia you can use Ld spell on fury units or double fiends for example to stop their lances totally while timewarp helps to counter lances even better. You need good fast armies that can hit well. +D6 charge banner on Letter horde with unit of 6 fiends and greater daemon is pretty good start for list building.

I would recommend a unit of 6 Crushers with the +d6 charge banner to suprise some cavalry units and a GD of choice, I find having both mitigates the preposterous amount of fire said opponents could levy on your Bloodthirster/Keeper.
That being said 2 out of 3 of those armies suffer from lackluster Ld. May I recommend a Regenerating Kipper with Lv.4 and some good ol' fashioned Despair Icon, remember his pain in the ass detachments will be using the Steadfast of their parent unit...