Is 12 cups equal to 3 quarts?

Yes it is.

1 quart = 4 cups And 1 cup = 0.25 quart.

There are 32 US fluid ounces (2 pints, or 4 cups) in 1 US quart (946.4 ml).

There are 40 Imperial fluid ounces (2 pints) in 1 Imperial quart (1,136 ml).

Note that there is no 'cup' in the Imperial system, it is best to use pints or half pints, and not 'cups'.