You need to think in a more structured way.
Your system needs 3 modes. (off, on, flashing)
Define a variable to remember which mode you're in.
Write code that increments this variable when you press the button and goes back to the first mode when necessary.
Use Serial.print statements to verify it's working.
Then write the code for each mode, that makes the LED do what you need.
I don't want to tell you too much, for the good of your education!
Good luck!
