there's no problem for the 3rd resource, because warcraft3 already uses 3.. you just have to change them, use food as the third one, but you'll have to trigger some stuff to make it fully work, like place some mining ores somewhere and trigger the gather of that ores, and add food.. the only problem is in buying stuff with that, because food is permanent, if you use it for units, the resource will be reduced by lets say 3, and it regains that 3 points if the unit die, so you have to trigger this part too.. the only advantage of this is, that you can look up all 3 resources without using a multiboard..
if you're going to use more resources you'll need a multiboard for it, and I suggest, that you show all resources in that board, and change the common gold and lumber icons and texts with something other.. like the date, or game time, or kills, whatever.. just to get a better overview. I guess it's better if you just open the multiboard and all important information is given..