Students in a science class measured rain by writing down the depth of water in a bucket that collected the rain. The bucket was empty before the rain began to fall at a steady rate over a 6
6
-hour period during school. After
3
3
hours, the water had a depth of
18
18
millimeters.

At what average unit rate, in millimeters per hour, did the bucket fill after rain started to fall at a steady rate?