Three cups of beer. Three kilograms of bananas.

3 cups of beer 3 kilograms of bananas Because bananas are quantifiable, countable objects.

"Three glasses of beer." OR "Three beers." "Cups" are small, have handles, and hold about 200 milliliters. In the U.S. we don't drink beer in cups. We drink coffee and tea in cups. We sometimes drink beer in "mugs," which are like cups but much bigger.

You could say 3 kilograms of banana if it was prepared banana to be used in a cooking recipe.

You could say 3 cups of beers if you wanted people to know that you had 3 different kinds of beer.