It can be done with 90% efficiency.
You can not convert the transformer without cutting the secondary windings. The secondary winding is a split secondary that is wound that way from the manufacturer. To accomplish what you asked would entail unwinding each side of the 12 volt winding half way. At this point solder a pigtail out to a terminal block. This would be your 6 volt tap. Then the coil would have to be rewound to its original state as before the unwind. This same procedure will then be done to the other side of the secondary. Done as a project, maybe, workable within ones capabilities, no.
If it is AC then you don't need a formula, you just need a transformer.
No. You need 12 volt AC to run a 12 volt AC motor, not 12 volt DC.
This might be a homework question, in which case you find the power, which is 3 x 746 watts, and divide by the voltage to find the current taken if the efficiency is 100%, and that is 9.325 amps. So the efficiency is 9.325/12 or 77.7%. In practice the question is complicated by the power factor of the motor, which means that the watts taken are less than the volts times amps. The power factor could well be 0.777, in which case the efficiency would be 100%.
12 volt
If that 1940 Chrysler is in original condition, it would have a 6 volt system. Originally, it was a 6-volt. However, many people convert them to 12-volt systems for convenience purposes.
Use a voltage devider or a transformer.
With an appropriately rated DC to DC converter.
Probably about 180 watts, assuming 90% efficiency.
change all the light bulbs to 12v
If it is a 12 volt panel it will light a 12 volt bulb. Most likely it is not a 12 volt panel, it is some other voltage, so you then need equipment like an inverter to convert the energy to 12-volt energy.
All you have to do is change the bulbs to 12 volts and supply the trailer with a 12 volt power source. If you leave the original 24 volt bulb in the trailer they will only glow at 1/2 of their rated wattage.
On would use a 12 volt battery for a lot of things. For example, a lot of Fisher Price toys and games, power wheels vehicles have a type of 12 volt system.
You can not convert the transformer without cutting the secondary windings. The secondary winding is a split secondary that is wound that way from the manufacturer. To accomplish what you asked would entail unwinding each side of the 12 volt winding half way. At this point solder a pigtail out to a terminal block. This would be your 6 volt tap. Then the coil would have to be rewound to its original state as before the unwind. This same procedure will then be done to the other side of the secondary. Done as a project, maybe, workable within ones capabilities, no.
Originally it would have been a 6 volt system.
No! You would damage it beyond repair.
If it is AC then you don't need a formula, you just need a transformer.