Three considerations if the OP's question is how do they work:
A heavier weighted back blade is more efficient for working dirt than a lighter one. There is typically no down pressure applied by a 3 point hitch, so increasing the weight of the implement is critical for ground engagement. You might find ways to add weight after the fact, but that can negatively affect a lighter built implement.
Depending on the intended use, being able to angle the main beam side to side at the 3 point hitch end adds considerably to the tool's functionality.
Third point is sizing the blade to your tractor. Generally, people recommend going as large and heavy as possible based on your tractor specs. A too large and heavy of a blade loaded with spoils can stop your movement if it exceeds the weight and power of your tractor. An undersized blade is better used as a garden decoration or boat anchor.