The group's president, Dhairyashil Patil, said the letter was sent to Reckitt, Hindustan Unilever, Colgate and 20 other consumer goods companies.
None of the three consumer companies, as well as Reliance, responded to requests for comment.
Mom-and-pop stores, or "kiranas", account for 80% of a near-$900 billion retail market in India. About 300,000 such stores in 150 cities order goods from Reliance, with the company setting a target of 10 million partner stores by 2024.
Traditional distributors have told Reuters they have been forced to cut vehicle fleet and staff as their business has been suffering because they can't match Reliance's pricing.