If the period of the pendulum is tripled, what is the length of the string increased by?

The period of the pendulum doesn't determine the length of the string. 
It's the other way around.

The period of the pendulum is proportional to the square root of its length.
So if you want to triple the period, you have to make the string nine times
as long as it is now.

Since the pully is a simple machine so the mechanical advantage is a 1