the bible stated that Jesus is God only son, and then it stated that Adam is God's son.
The Bible doesn't actually say that Jesus is God's only son. It says He was God's Only "Begotten" Son. God is referred to in the Bible as the "Father of spirits." We (all of us) are referred to as "His offspring." Thus, even though God is the Father of all of our spirits, He is the literal Father of Jesus' mortal body as well as the Father of His spirit. They have a literal Father-Son relationship, just as you have a literal father-son relationship with your own father.
Again, a person could easily say
1 - Adam sinned but was not physically punished - why?
I'm not sure why you believe Adam was not physically punished. Don't you see expulsion from Eden as a pretty significant hit? Had Adam not transgressed, he would have lived in Eden forever. I'm sure he saw being forced into the world outside of Eden as a punishment.
2 - Jesus was physically punished but he never sinned - why?
He was punished because He agreed to take upon himself the guilt for all of our sins. Being sinless himself, He was the only one who was in the position to be able to do so.
3 - Jesus was baptized - which is the washing away of sins - but he never sinned - why?
The only thing the Bible tells us about the reason Jesus was baptized was that it was required of Him "to fulfill all righteousness." In other words, He was doing what He knew God expected of Him.
3) was it that the Jesus's physical punishment was for the sins he committed while he was Adam?
It couldn't have been, because Jesus wasn't Adam.