Citroen C 3 which end is number 3 cylinder

If you are going to do this yourself do it with the engine COLD. Coolant operate at over 100'C so it will burn you!
I think on that engine the sensor and thermostat are built as a single combined thermostat housing. The easiest way to find the Thermostat housing is to follow the large, black coolant pipe that is coming out of the top of the radiator. follow it back towards the engine and the other end of the hose should be attached to the thermostat housing.
On the 1.6 it is usually on the transmission end of the engine between the cylinder head and the air filter box housing which you will need to remove. Once you have removed the air filter box access to the housing is much easier.
I have attached a video link as a guide for the job. The car in the video may be different to yours but the 1.6 is a standard Citroen engine so it is fitted to lots of different models.

I think the Deutz F3L 1011F engine is a 3 cylinder engine.So it fires as 1, 2, 3. Number 1 is the cylinder nearest the belts and pulleys end of the engine.

Not knowing what the actual problem is I can only suggest the firing orders.
Is yours the four cylinder engine or the six cylinder?
The Four cylinder engine will probably use the sequence 1, 3, 4, 2, as this is usually the most common one in use. Cylinder One is the cylinder nearest the belts and pulleys and you count away from you towards the transmission and gearbox.
The six cylinder uses the firing order 1, 2, 3, 4, 5, 6. Cylinder One is also at the belt and pulley end of the engine but the numbering is different on a 'V' engine. The numbers go 'left, right, left, right, left, right' odd one side evens the other.
Stand next to the engine with the gearbox furthest away from you and the end with the belts and pulley wheels nearest to you. Number One is the cylinder nearest to you on the Left. Number Two is the one nearest to you on the Right. The Left side is numbers 1, 3, and 5 and the right side is 2, 4, and 6,

Firing order is correct, (1, 3, 4, 2). Cylinders are numbered as 1, 2, 3, 4. Number One cylinder is nearest the belts and pulleys and number 4 is nearest the transmission/gearbox end. Stand in front of the car and face the rear of the vehicle. Cylinder 1 is to your left and Cylinder 4 is to your right.

Standing at the front of the engine,
the cylinders on your left are 1 - 3 - 5
the cylinders on your right are 2 - 4 - 6
With 1 & 2 being the closest to you.
5 & 6 are at the back.

Like so...

This is where it gets tricky, Graham, the French have started calling the flywheel end, cylinder no 1, on some engines. As the timing is not adjustable is it just that you need to know which lead goes where?
You can see the no 1 position usually through a hole in the bellhousing which is marked on the flywheel. A blob of white paint or Tippex can help highlight it.

I don't know this car, but the #1 cylinder is always at the front of the engine. The front is where the crankshaft pulley turns the serpentine belt or all of the belts that are utilized, and the rear of an engine is where the transaxle mates to it. So the cylinder numbering order is 1-2-3-4.
The firing order on all 4 cylinder engines is 1-3-4-2.

One thing you didn't say you replaced was the plug wire. That could also be the problem.

this engine is turned sideways in the engine compartment. looking at the front of the engine, where belts and pulleys are mounted, the odd number cylinders are on the left, the even to the right. starting from the front of the engine, counting towards the rear, the cylinders are numbered 1, 3, 5 on the left and 2,4,6 on the right. remove the spark plug in the number one cylinder and place a peice of tissue in the hole. now crank the engine in very short bursts. as soon as you hear the engine crank, let off the key. check for the tissue to fly out of the spark plug hole. once the tissue flies out, the number one piston is up. now you need to take the cap off the coil, and see where the rotor is pointed. hold the cap over the rotor to see which terminal lines up with the rotor contactor. connect one end of the plug wire to the corresponding terminal, and the other end to the number one cylinder spark plug after it is reinstalled. from here the coil terminals are numbered in a clockwise direction. the firing order for your engine is 1-2-3-4-5-6, so you just connect one end of a plug wire to the number two terminal, and the other end to the spark plug in the number two cylinder. repeat this procedure for the remaining wires,

all engines r timed off cylinder no 1 and that is always the cylinder at the front of the engine (the end where the alternator belt is) hope this helps