Pour the cans of condensed milk into the 3 canning jars, dividing evenly. Each jar should be practically full.
Seal each jar tightly with a ring and lid.
Place jars into the crock pot. Pour in enough water so that it is about 1/2-inch below the lids.
Cook on low for 10 hours. Remove jars from water.
It will be thick like a custard, but when stirred with a spoon, will resemble a thick caramel sauce. For an even thinner consistency, Ann suggests shortening the cooking time to about 8 hours. Store in refrigerator.