How much is trimalleolar fracture curable by insertion of metal plates and screws?

Very. This one is among the severest of fractures; and the 'hardware' is placed to stabilize the joint so it can help the joint heal.
Close to a cure... A trimalleolar ankle fracture if displaced, meaning the joint is not aligned and the fracture pieces are not in contact with one another, needs surgery to restore ankle stability, joint alignment. The fracture will typically heal when fixed with screws and a plate or two. While the bone typically heals, these fractures also impart cartilage injury to the ankle joint which typically do not heal.