The job of fielding at third man is generally seen as damage limitation. In other words, it's all about saving runs.
You will find the third man positioned behind the wicketkeeper on the off-side.
The fielder is usually 45 degrees to the wicket around on the boundary.
It covers a large area - anything that goes through the slip and gully area.
Often Test teams don't have a third man because they prefer to attack than defend.