Three things that would make a sledge unusable are a ? head, a loose head, or a damaged handle.

A rounded head on a sledge can significantly impair its effectiveness. A flat head is typically preferred for sledgehammers, as it allows for efficient striking and hammering of objects. A rounded head may lack the proper surface area required to deliver powerful, direct blows, which is essential in both driving stakes and breaking materials. Moreover, the rounded shape can result in glancing blows rather than a solid impact, thereby risking safety and making the tool less effective for its intended use.

In contrast, the other types of heads, such as flat or tempered ones, are designed for specific tasks and maintain the necessary characteristics that ensure proper functionality and durability. A soft head will also not provide the required hardness for effective use, but it would not be considered 'rounded' like the choice in question. Each of these factors contributes to the overall performance and safety of using a sledge.