In Fire Emblem Three Houses, tending to your weapons is very important before heading off into battle as they degrade over time. There is a Blacksmith in the game, but it isn’t available until a few hours into the game. You’ll unlock the ability to use the Blacksmith after the “Assault at the Rite of Rebirth” mission, where you will then get a quest in the Marketplace (furthest southern part of Monastery) area from the Blacksmith.
The Blacksmith will take some time to unlock
This Quest is called “Forge the Way” and it will require that you track down iron welding materials and bring them back to the Blacksmith to unlock their services (as well as some gold and item rewards). The iron welding materials are right at the top of the stairs at the marketplace, not very far from the Blacksmith. Simply head over to the blue glowing area, grab the materials and head right back to the Blacksmith to unlock it.
Once you have the Blacksmith unlocked you will be able to not only repair the weapons that you own, but you can forge weapons at this vendor. Depending on whether you’re trying to repair or forge weapons, you’ll need resources and cash to do so. Forging different weapons also requires that you have a certain professor level to do so, depending on what weapon you are trying to forge.
The Blacksmith can also repair Rusted Weapons that you find. These weapons can be of different values and that can’t be determined until you take it to the Blacksmith to repair it.