Fallout 3 crafting

Basics
Schematics allows one to create homemade weapons at a workbench if they have the proper components. Acquiring multiple copies of the same schematic allows the player character to make multiple copies of a weapon from one set of components (bottlecap mine, Nuka-grenade) or weapons with higher initial condition at the time of construction (all other weapons).

Multiple copies of a schematic
Obtaining more than one copy of a schematic upgrades the plans. Additional schematics cease to yield benefits after three of each kind have been located; they will simply appear as items in your inventory.

For non-explosives, two plans increase the initial condition of the weapon at construction to 100% of the player character's repair skill (from 75% of your repair skill with just one blueprint). Three plans net a condition of 125% of one's repair skill at construction; however, a weapon cannot be built with a higher starting condition than 100%.

For the bottlecap mine and Nuka-grenade, additional schematics allows more items to be crafted from a single set of components instead - one additional item per set of schematics, for each set of components.

Repair skill
Higher repair allows one to make homemade weapons that have higher starting condition and damage. As the player character makes multiple copies of the same weapon they can combine them with the repair skill for higher condition and damage. Alternatively a non-player character can repair them. The bottlecap mine and Nuka-grenade are always at maximum condition.