What country is the third largest?

The U.S.A., demographically speaking.
In terms of area size, the 3rd largest country in the world is China. In terms of population, the third largest country is the United States.
china
The world's third largest country is the United States of America in both land mass and population, being 9,629,091 sq km in size, containing 310,681,000 people and making up 4.52% of the world's population.
By population and area, the US.
By total area, the United States is the 3rd largest country in the world with an area of 9,826,675 km2.
1) Russia - 17,098,242 sq KM

2) Canada - 9,984, 670 sq KM

3) The US - 9, 826, 675 sq KM

4) China - 9, 596, 961, sq KM