Hello Bharath,
All of the three courses are incredibly intensive with a lot to learn. It depends upon your interest. Aerospace engineering is about building and designing aircrafts and spacecrafts. Aeronautical engineering would focus solely on vehicles that fly within the earth's atmosphere. So, essentially Aeronautical engineering is an specialized form of Aerospace engineering.
Astrophysics is a different field altogether. It is concerned with the stars, their physical nature, application of laws of Physics to study the observations and other celestial bodies.
So, it's on you to figure out if you actually want to study about stars and planets and planetary motions, in which case go for Astrophysics.
If you want to study about and work on building aircrafts and spacecrafts, you can opt for Aerospace engineering.
Hope this helps. Good luck.