The sound of 'er' is common in them.
Explanation:
The sound that one can find common in all of the examples is the insistence on the sound of the letter 'e' as it is used int he same way in the three letters that is how it is used to as a vowel.
This makes the three words seem connected in a sort of alliteration due to the repeat of the sound of the vowels that constitute the words.
This also leads to some reliance upon the word or sound of 'er' which can also be evidently seen here that is there in all three of the words given.
