Answer
One-third ($\frac{1}{3}$) of the tank is filled within one hour.
Work Step by Step
Given that the whole tank fills within three hours.
Thus, if we divide the entire tank into three equal parts, then every part gets filled up in an hour.
(Thus, three parts take one hour each, taking the total time to fill the tank to three hours.)
Thus in an hour, One-third ($\frac{1}{3}$) of the tank is filled up.
