"I have resigned online which they accepted retrospectively condition of 1 month salary and any outstanding dues (which i considered nil)."
Have they given written acceptance of resignation?
What you consider about o/s dues is alright, but what is the record of PSU about loans, advances etc
You will have to write query more clearly and chronologically for better answers.
Why you waited for 3 years is also not clear?
16th August 2019 From India, Pune
I had to withdraw my savings from their trust so I approached them. Even today while asking for demand letter they got annoyed on the mention of demand letter.
22nd August 2019 From India, Delhi