I don't know if "fruit" works as a singular or a plural in these cases: HOW MANY fruit(s) or HOW MUCH fruit do you eat a day? Today I have had three fruit(s).
When the question
How much fruit do you eat in a day?
is asked, although the noun fruit is uncountable in this context, it is idiomatic to use a countable noun when you respond (as James K has already touched upon)
I eat three pieces of fruit every day.
The same applies to other uncountable nouns, such as money:
Q: How much money do you have?
A: I have three dollars.