Do I need the experiecne letter from all the previous employers for filling I140?

[ZAHEERslc]

My PERM approved last month,I have managed to get the experience letter from 2 out of 3 of my previous employer.One of them is not responding to my emails. 
The perm states 24 months of experience required.The experience letter from the two employers covers 8 years of experience of the skills mentioned in PERM. 
Do I need to get the experience letter from the 3rd employer as well?

[NotAnAttorney]

Okay, I assume that your petition is being filed in EB3 category.

You will definitely not need from all your past employers.

The main purpose is to prove that you have so many years of experience in the desired skills that were published in the PERM.