Conditional Averaging

Hi, newbie here. I am trying to figure out how to take the average of one column, dependent on it meeting the criteria of TWO other columns

For instance:

Professor Course Number Difficulty
Abe 101 5
Abe 101 3
Abe 101 5
Abernathy 101 2
Abernathy 101 4

I want to be able to create a function that only takes the average in column 3 (Difficulty) when Professor is Abe and course Number is 101. I tried to use the average if function, but keep getting errors. Thoughts?