Dear Bhaumick,
I will start with following Note from Document Checklist which says:
NOTE: If you are unable to provide any of the requested documentation, include with your application, a written explanation with full details as to why that documentation is unavailable and any documentation that would support your claim. If your documentation lacks any of the documents without a reasonable justification, it will be returned to you, or in certain circumstances could result in the refusal of your application.
Having referred that, let's come back to you question now. Your case is similar to my case except two things:
1. I was not able to provide reference letters therefore I wrote explanation for my all 3 employments stating why reference letters are not available and what I am providing as alternatives. These explanations were written to comply with the NOTE in Document Checklist so that my application shouldn't be returned (as incomplete) or refused (as not eligible).
2. One of my previous employer has written one sentence about my major job functions (designed, installed and commissioned) on the experience letter and others required information was also available in different official documents. Therefore, I referred those document as alternatives of reference letter in explanation.
Your application has to clear two phases; completeness and eligibility. It totally depends on Visa Officer if he/she considers your application complete without an explanation (as reference letters are missing) and eligible without job description & responsibilities (as these are not declared by employers).
I wish you good luck buddy.
