The combining form -ful is used to form nouns meaning ‘the amount needed to fill’ ( cupful, spoonful, etc.). The plural form of such words is cupfuls, spoonfuls, etc. Three cups full would denote the individual cups rather than a quantity measured in cups: on the sill were three cups full of milk; add three cupfuls of milk to the batter.
Definition of -ful in:
- The British & World English dictionary