You must make your claim on or before 29 January 2021.
To make a claim for the third grant your business must have been impacted by coronavirus on or after 1 November 2020.
You can make a claim for the third grant if you’re eligible, even if you did not make a claim for the first or second
https://www.gov.uk/guidance/claim-a-grant-through-the-self-employment-income-support-scheme