Make all 3 to get the best leveling progress

Levelling it would only be worth it to trade weapons. Farming weapons is another story... I made a titan and hunter today just to get the x 3 bonus drop buff for the menagerie. If you have the one character you only get 1 x 3 buff so 6 - 9 drops. So therefore with three characters 18 - 27 chances at getting a perfect rolled item for the week. I was chasing an imperial decree with, rifled barrel , accurized rounds, slideshot, rampage, and a range masterwork. I got it and the decision to make the titan and hunter again was well worth it. They will act as the farming mules and I’ll main my warlock for everything else.

You'll be better off making one of each class. Bungie decided that they didn't want you to play on multiple characters of the same class, so they nerfed loot drops so that only the first character of each class can claim powerful engrams each week. If you have three warlocks, for example, only the first one will receive powerful or pinnacle engrams from weekly activities. The second and third warlock will only receive legendary drops from the weekly activities which will not help boost your power level past the soft cap. The only reason to have multiple characters of the same class now is if you really enjoy playing on that class and want to play through all the activities on the same class. The trade off is that it will take you longer to reach the hard cap than players who play on multiple classes. Edit: I suppose if you really wanted to farm raid gear for a class it would be better to make multiple characters of the same class. You could run the raids on each to get drops, but only one would get powerful drops.