Dual employment

better to consult local labour oriented lawyer and get clarity

Shed down all such doubts and fears as no one bothers about the past that has taken place three years for the very old employer.  This is not a very serious things and you can orally explain such things to company 'C'

Have you supressed the relevant information from the prospective employer?

You should have tendered resignation letter to company A at the time of leaving that company, since you have not followed the proper procedures, you are made to worry about such issues, which may be petty or may create problem for your future career too.

In any case you can join the company C based on the proper relieving from company B and and can give an explanation explaining the circumstances which made you to leave the company A at that time, if you are given a show cause notice by company C in this regard.

Since your tenure of service with the company A was for a very brief period, you can convince the reasons for leaving the company without informing them.