At some point Maegor the Third tried to outlaw the practice of usury, and cut off both the hands of anyone caught charging interest on loans.
In the books
In the A Song of Ice and Fire novels, there is no such character. Indeed, there has never been any other king of House Targaryen named Maegor since the infamous "Maegor the Cruel", such was Maegor's repute that later members of the house deliberately shunned the name. So far, it is unknown if the mention of the name is a deliberate choice by the production (such as intentionally depicting Mace Tyrell as uneducated for believing there ever was a Maegor III), a mistake from the writers, or a mistake from the actor (the name may have been confused with Maegor the First, who was known for cruel policies and it would not have been uncharacteristic of him to attempt to outlaw usury in a harsh manner).