Who was Radius helping when he discovered that the "magic number was more than 3"?

Radius was helping his father Sir Cumference.

Explanation:

The question above is related to the mathematical adventure story entitled "Sir Cumference and the Dragon of Pi."

Radius was the son of Sir Cumference and Lady Di of Ameter. One day, Sir Cumference drank a potion which turned him into a dragon. In order to restore his father's shape, radius needed to find the magic number called "pi."  

In mathematics, the value of pie is more than 3 (3.14 to be exact) and it is represented by the Greek symbol "."

So, this explains the answer.