Class III amalgam restorations are primarily indicated for carious lesions on the proximal surfaces of anterior teeth, typically involving incisors and canines. The distal surfaces of canines are a common site due to their position adjacent to the premolars and higher susceptibility to plaque accumulation. These restorations address cavities without involving the incisal edge, preserving tooth structure and strength. The choice of amalgam is based on durability and ease of placement in these areas where isolation can be challenging. Therefore, distal surfaces of canines represent a frequent and appropriate indication for Class III amalgam restorations.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.