aptitude

If there are 4 fruits and you guess 3 of them correctly then you will have guessed the remaining fruit correctly as well.

So 0.

But you could look at it the other way saying that if you guessed all 4 correctly then you have me the required 3 fruits so all the rest of the poeple will have guessed 3 correctly, it's like the "how many months of the year have 28 days?" they all do, just some have more than 28. So the answer could be 10

Nobody. Cause if they guess 3 they automatically guessed the 4th one so.... nobody.

Or maybe 31?

This is a problem of " inclusion-exclusion principle " . Robert Gentel was right in realizing that if one has guessed 3 fruits correctly then that person
has also guessed 4 fruits correctly ; tough Gentel ' s answer was wrong .
If y number of people have guessed 3 fruits correctly then ,
123 = 43 + 39 - 31 + 4y - y i.e. 3y = 72 i.e. y = 24 is the answer .