I started with one, played a holy pally. I hated the slowness of kills, QD dailies took me way too long, but I couldn't stand the thought of solo leveling again (It took me forever and a day to get to 70 the first time) So I decided to boost up a DPS toon to go along with my holy pally, so I rolled a hunter on a second account, got him to about 30 before i wanted to stab my eyes out. (this was before I knew about things like keyclone...) I tried a couple other dual boxing combos, warrior and second paladin, priest mage, druid shaman... Never got past lvl 10 on many of those.

Then I discovered keyclone, this website, and the idea of 5 boxing. So I figured, i'd end up with 5 eventually, so I bought 3 more battlechests, and started my journey. I found leveling a 5 man team much more fun and challenging. My first team was all mixed classes, paladin tank, holy priest, boomkin, ele shaman, fire mage. I picked it for buff synergy, and I got to about level 25 before I went for an easier team of 2 classes, 1 healer 4 dps.

My opinion? Go right to 5 (4 if you have friends). WIth 5, you can do all the non raid content, never have to wait for a group, etc.

But if you stick to just 3, I'd pick 2 hybrids, and 1 pure dps class. Like Druid, Paladin, and Hunter/Lock. Having the pet from the hunter/lock helps a bit when boxing, as the pet can be semi autonomous. I know my felguards on my lock team help out a bunch, like when one lock gets stunned and knocked off his mount, and I dont notice, the pet keeps the mob busy while I double back. I bet if you played a ret paladin main, with a boomkin and a lock, you would be very powerful. And you'd have the option to respec and make a pure tank, pure healer, and only have to PuG for DPS, which are easy to find.