Arduino will not read three successive packets in the serial connection
The problem is that your code works but you substract 48 from what you received. 
Which if python sends a char with values: '0' '1' '2' it will result in the serially outputted values of NUL SOH STX which are invisible.

If python transmits the raw byte value (aka 0x00 0x01 0x02) this would result in 0x00 - 48 = undefined behaviour with a underflow (same goes for 0x01 and 0x02)