3 bananas + 2 oranges,then can I say there are 5 fruits,or do I have to say 5 pieces of fruit?
My confusion here is whether fruit is countable or not.

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Answer

You would say 5 pieces of fruit. 
'I have 5 pieces of fruit. I have 2 bananas and 3 oranges.' 
'How many apples do you have?'
'I have none. I have 5 pieces of fruit, none of which are apples'

Answer

You say "five pieces of fruit".  Fruit is a collective uncountable noun here.

If you say "fruits" (which is uncommon), then you mean "different types of fruit".  In your example, you have two fruits: banana, and orange.

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