Explanation:

- Option 1: 3 October

- This is not the statehood day of Tripura.

- No significant event related to Tripura is observed on this date.

- Option 2: 12 March

- This date is not related to Tripura's statehood day.

- It could coincide with other events, but not relevant to Tripura's statehood.

- Option 3: 21 January

- This is the correct date.

- Tripura, along with Manipur and Meghalaya, was granted statehood on January 21, 1972.

- Option 4: 21 February

- This is not the statehood day for Tripura.

- February 21 is observed as International Mother Language Day, which is unrelated to Tripura's statehood.