3 locks is quite viable, but i guess depends on what you actually intend to do. You won't be able to 'solo' any end game dungeons with them. You'll probably enjoy leveling / pvping on them though

Give ragefire chasm a go since it's lower and alot easier for you , just to practice. If you use the auto queue system (press i ingame) it'll queue you up with a random tank and healer. Be prepared for some to not be 'acceptive' of a multiboxer though. This caused me a fair amount of trouble initially and is why I 'graduated' to a 5box team so that I didn't have to rely on other people for groups.