Fallout 3 crafting

Basics
Schematics allow you to create homemade weapons at a workbench if you have the proper components. Acquiring multiple copies of the same schematic allows you 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. Two plans makes the initial condition of the weapon at construction equal to your repair skill (75% of your repair skill with just one blueprint). Three plans will net a condition of 125% of your repair skill at construction.

Additional schematics for the Bottlecap Mine and Nuka Grenade allow you to make more items from a single set of components instead of increasing the initial item condition. With one schematic - you make one. With two - you make two. With three - you make three.

Additional schematics cease to yield benefits after three of each kind have been located. They will simply appear as items in your inventory. Also, the produced weapon's condition is still capped at 100%; A third-level schematic with a Repair skill of 100 will still produce weapons at 100% condition, NOT 125%. As such, the third schematic becomes pointless once Repair is elevated to 100 (Bottlecap Mines and Nuka Grenades excepted).

Contrary to the in-game text, additional Schematics do not increase damage. As the base item is the same, the base statistics are as well. The only change is in its condition (or number, for the Bottlecap Mine and Nuka-Grenade).

Repair skill
Higher Repair lets you to make homemade weapons that have higher starting condition and damage. As you make multiple copies of the same weapon you can combine them with the repair skill for higher condition and damage. Alternatively you can have an NPC repair them. The Bottlecap Mine and Nuka Grenade are always at maximum condition.

Appearances
Schematics appear only in Fallout 3.