The term "astronaut" refers to a person who is trained to travel and work in space. Let's look at the options:
- Astronomer: Refers to a scientist who studies stars, planets, and other celestial bodies, but not a person who travels in space.
- Astrologer: Refers to someone who studies or interprets the influence of celestial bodies on human affairs, unrelated to space travel.
- Astronaut: The correct term for someone who travels into space.
- Jetsetter: Refers to someone who travels frequently, but not necessarily into space.
Therefore, the correct answer is "astronaut".
\[
\boxed{3. \, astronaut}
\]