When shoveling gravel using a Fortune III enchantment, the drop rate for flint isn't 100%. Just make a 1x60 tower, then use your enchanted shovel towards the bottom of it, there is a chance of gravel drops. The drop rate for flint should be 100% for this enchantment as stated in the wiki. This was tested in Survival mode.
Minecraft wiki: http://minecraft.gamepedia.com/Flint