i have never heard of sensor 3 personally u can have 4 sensors fitted i.e for a v6 etc but usually it will be broken down as sensor 1 bank 1 or sensor 2 bank 1 then sensor 1 bank 2 and sensor 2 bank 2 bank 1 would be for example cylinders 1,3,5 and bank 2 for 2,3and 6