NO!
A twelve volt charger will charge twelve volt batteries only unless it has a feature that allows you to change the chargers voltage. But on fast or slow charge 12 volts is 12 volts so most 12 volt chargers are only good for 12 volt batteries.
24 Volts.
You will have a 12 Volt source with double the charge capacity.
Two six volt batteries in series makes 12 volts so switch the charger to the 12 volt position.
how Can you charge multiple 12 volt batteries with a car?
No, but you can charge each one individually without separating them from the series loop.
No, if they are wired in series you have 36 volts assuming each battery is a 12 volt battery. They will have to be wired in parallel to output 12 volts. In that case you can charge them with a 12 volt charger but it will take 3 times as long as charging just one 12 volt battery.
Modern automobile batteries are 12 volts.
All automobile batteries are considered 12 volt batteries. At the battery itself. With a digital volt meter set of the 20 Volt DC scale you should read 12.6 volts on a fully charged battery with the engine not running. You will see 12.4 Volts at 75% charge, 12.2 Volts at 50% charge and 12 Volts at a 25% charge. With the engine running you should read from 13.5 to 15.5 volts if the alternator is working properly.
A 10 amp charge will only output 10 amps and 12 volts. So, if you want to charge thee 12 volt batteries hooked in series you will have to disconnect the positive cable from each battery and charge then individually. You cannot charge three 12 volt batteries hooked in series. That requires a 36 volt charger.
they are 12 volts
You must hook them up in series. You will need four 12 volt batteries to do this.