On December 8th 2020, Starship SN8 was the first SpaceX Starship prototype to fly to a height of at least 1 km (12.5 km to be specific). SpaceX intends to one day use Starship to bring humans to Mars.
When will the third SpaceX Starship flight be?
This question resolves to the time of the third Starship flight, with the December SN8 flight considered to be the first. The criteria for "flight" are the same as in this question, notably, the Starship must reach a height of at least 1 km intact for resolution, and what counts as a Starship is outlined in this other question.
This question has a sister, asking about the 2nd Starship flight.