The input to the ADC is selected by the four least-significant bits of
the “ADC Multiplexer Selection Register”, aka ADMUX. It is set to
analog input 3 by this line of code:
ADMUX = 0x03;
Change this line to ADMUX = 0; and the ADC will read pin A0.
Your code is to hard to read, it is just a blob of characters on my screen but you have a pretty picture of the arduino. Change your code to use A0 instead of A3. Change your physical connection from A3 to A0 and you are all set. I miss read the code you posted.
It is a tricky circuit and he did all of the the registers directly, making it extremely hard to ...