Let 

x = the age of Pete

y = the age of Bill

Pete is 3 times as old as Bill means:

x = 3y

The sum of their ages is 48 means:

x + y = 48

We have the following system of equations:

x = 3y

x + y = 48

........

click here to see the system of equations solved for x and y

........

x = 36 years

y = 12 years

Pete is 36 years old.

Bill is 12 years old.