Connecting 100 DC motors (3V) to a battery or mains
Here are the specs of your motor:-
3V Rated D.C. Miniature Reversible Model Motor, 2mm Shaft, 1.5V to 4.5V
No Load Start Voltage         0.6V     
Rated voltage                 3.0V
No load current               0.34A
No load speed                 16400 rpm nominal
Rated load                    8 g.cm
Rated load current            1.07A
Rated load speed              13100 rpm nominal
Length excluding shaft        25mm
Diameter                      20mm
Width across flats            15.1mm
Shaft length                  9.4mm
Shaft diameter                2 mm
Weight                        17g
Magnet Type                   Ferrite

We can calculate the internal resistance from the change in rpm and current between no-load and rated load, (3.0 V - (3.0 V * 13100 rpm / 16400 rpm)) / (1.07 A - 0.34 A) = 0.83 Ω. Then the stall current is 3.0 V / 0.83 Ω = ~3.6 A.
Depending on mechanical load the operating current should be somewhere between 0.34 A and 1.07 A. If we assume an operating current of 0.7 A then to power 100 motors simultaneously your 3 V power supply would have to deliver 70 A continuous and up to 360 A peak (perhaps a bit less due to resistance of the wires going to the motors).
To use a car battery you need a way to reduce the voltage from 12 V to 3 V. This could be done with small DC/DC 'buck' converters such as this. As the converter 'transforms' the voltage down it increases current output by the same ratio (less ~10% conversion loss), so a motor current of 0.7 A would only draw ~0.2 A from the battery.
If the DC/DC converter is mounted on or close to the motor then the power supply wiring can be thinner because it doesn't have to carry as much current. However the converter draws current in pulses, so you should then add a large electrolytic capacitor (eg. 220uF) across the converter input to eliminate voltage spikes caused by the wiring inductance.
Using DC/DC converters the 12 V car battery would have to supply 0.2 A * 100 = 20 A, which is roughly equivalent to four 55 W headlights. Whilst it should be able to do it, the battery would not last long at this current because car batteries are not designed for continuous drain at high current.
However high current 12 V power supplies are readily available, so something like this would be a better solution if mains power is available. Alternatively you could use several smaller power supplies each powering one bank of motors, eg. four 12 V  10 A supplies powering 25 motors each. Either way the power supply should be rated for higher current for cooler running and so it doesn't shut down during the higher motor startup current.