What is fortune 3 used for on minecraft?

I'm going to guess that you're using this on a pickaxe. But here:

For coal, diamond, emerald, nether quartz and lapis lazuli, level I gives a 33% chance to multiply drops by 2 (averaging 33% increase), level II gives a chance to multiply drops by 2 or 3 (25% chance each, averaging 75% increase), and level III gives a chance to multiply drops by 2, 3 or 4 (20% chance each, averaging 120% increase).

in short, it increases the block rate. So if you had it on your pickaxe, and dug up a diamond ore, it could give you two diamonds instead of one.

Fortune potential increases the drop rate of the tool it is on. Unfortunately it only works on ores that drop their final form when broken. That is it has a chance to increase the number of items you gather when you use it to mine: Coal, Lapis Lazuli, Emerlads, Diamons, Nether Quartz, and Redstone.

It has no effect on ores you have to smelt, such as Iron and Gold.

I believe Fortune III is the highest tier or this particular enchant.

For its weapon-based companion, see the Looting Enchantment.