How much petrol is consumed by FAN, HEATER and AC in a petrol manual car?

Fan and heater take very little extra fuel, if any, since the only power consumer of power is the electric heater fan, and that is minimal. AC takes more, since the AC pump runs off the engine. But at worst, it might affect fuel consumption by one or two miles per gallon.

Blower fan probly 1/3 to 1/2 horsepower...

AC compressor probably about 5 or a little more.

You take the amp draw of the electrical accessories multiplied by the voltage, this will give you the watts.

1 watt = 0.00134102209 horsepower

The A/C compressor will have a rating for HP from the manufacturer so I can't answer that HP draw.

To actually figure the gas mileage, you would have to average it. So say the engine is 100 HP and gets 25 MPG; If you add 10 HP in accessory load, the 25 MPG mileage will drop by 10% or 2.5 MPG. Thus the overall mileage would drop to 22.5 MPG.