Definition: In this task, you need to reverse all words of a given length in the sentence. The number of letters in a word determine its length. For example, the length of the word "apple" is 5.
Input: Sentence: 'an adult skier is leading the way down the slope for children following behind'. Reverse all words of length '3' in the given sentence.

an adult skier is leading eht yaw down eht slope rof children following behind