Explanation:

- The 2020 Online FIDE Chess Olympiad was an unprecedented event held due to the COVID-19 pandemic, with matches conducted online.

- India and Russia were declared joint winners after the final match was disrupted by server issues, making it impossible to determine a clear victor.

- Option 1: The US – The United States did not win the 2020 Chess Olympiad.

- Option 2: Serbia – Serbia was not among the winners in this edition.

- Option 3: Russia – Correct, as they were co-champions with India.

- Option 4: Estonia – Estonia was not a winner in the 2020 Online Olympiad.