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: 'a lady standing on the roadside holding a pink umbrella with a large field and mountains behind her'. Reverse all words of length '3' in the given sentence.

a lady standing on eht roadside holding a pink umbrella with a large field dna mountains behind reh