I was thinking about your reply and it made me figure it out.
After pressing a button for the first time, the code enters the while(digitalRead(30)==0&&digitalRead(31)==0&&digitalRead(32)==0) loop, wich don't check for the serial.
The serial.available was reached only after pressing a button and exiting this loop.
So I put the check for the serial inside the while(digitalRead(30)...) and it worked.
Thanks for the help, it made all the difference.