Why are there 3 minor scales

There are 3 minor scales, or more precisely, 3 variants of the minor scale because of how harmony and melody interact in tonal music. Composers change some notes of the minor scale to achieve a specific sound for a particular style.

Does every song have a chorus?

No, not every song has a chorus. While most songs do have a chorus, there are plenty of great songs without one. These songs are just as effective and prove that is not necessary for a song to have a chorus.