My understanding is this: Fruit cup: A drink made from mixed fruit juices which may have a few small pieces of fruit as decoration. Fruit salad: A dessert composed of pieces of various fruits. Fruit cocktail: A slightly old-fashioned term for a fruit salad. If this is on a drinks menu, it could also be a non-alcoholic drink. May also be a flavour of e.g. yoghurt. This may vary depending on the country.

In my experience, all of them are commonly used. Fruit salad is probably the most common. Fruit cup implies that it comes in a cup (and contains fruit salad!)