"I buy three oranges."
Translation:Jag köper tre stycken apelsiner.
Yes, three individual oranges in this case. We like to use this expression when talking about things that are counted. Especially at the end of sentences, like this: - Hur många böcker köpte du? - Tre stycken. 'How many books did you buy? - Three.'
PS revisited this comment 8 months later and want to add this: I'm not sure if stycken is technically the plural of ett stycke. At least in my speech, the plural of stycke has a different tonal accent than this word. (the one meaning 'ones' has the acute accent.)