3 or 4 coals? That's nowhere near enough. One would need to "roughly" measure the interior by it's length (L), it's width(W), and it's height (H). One then multiplies all of those measurements together
(L x W x H). This produces the "volume" of the interior space. Then that volume is multiplied by .135 kg/m³. If you measure the interior in inches, you need to convert the inches to meters first, which you can do with online conversion calculators easily. After obtaining the interior volume in meters, you multiply it by the .135 kg/m³ I already mentioned, and since the m³ part cancels each other out, you will be left with a number in kg that is the amount of charcoal by weight that you should use as a MINIMUM. Whatever the minimum is multiply it by 4 for assurance.
Doing CO in a car can be tricky because of all the potential leakage places, like the vents, under the doors, around the door seals, behind the back seat. All kinds of places. There's a procedure for doing CO correctly that I implore you to investigate and read BEFORE you attempt it. You need to get the charcoals burning correctly outside the car until glowing red and no smoke, AND THEN place them in the car for a while to allow the CO to build up. If you get it wrong and don't get the CO to a high enough level, you can seriously injure yourself, maybe to the point of becoming a vegetable, but still alive, and in a "new" hell you might never be able to try and escape from ever again. You could end up in a nursing home being spoon fed and having your ass wiped. Not trying to scare you, just telling you how it is. CO is NOT a method you just try and do on a whim with no prior planning or research. You really need to take the time and investigate it and LEARN what to do and what not to do.