For ease of use, I'd really recommend you try to go with 1 class for your DPS, or at most 2. I'd probably double up one of the 3 classes you've picked, 2x mage or 2x lock. 3 different is going to be fine, but if you're looking to get into raids with your entire group, mages > hunters. Since you pretty much just spam frost bolt. Also, from my perspective, if I am a raid lead and I'm open to letting a multiboxer into the raid, I'd actually prefer them to be running all of the same class, or as many of the same class as possible for loot reasons. Only allow one character to get gear at a time, so people don't feel like "Oh, Mercbeast is trying to get loot on a hunter, warlock AND a mage!!".