150 surveys had some response.
responded to all three (this is an example of the inclusion/exclusion principle)
We start with the number of surveys with response subtract the numbers
of repsonses to each question, but now we have removed those that
responded to more than one question multiple times - twice for those that
answered exactly two questions and three times for those that responded
to all three.
So adding back in the numbers that responded to each pair of questions
we have removed those who answered exactly two questions, and left
one count in for each person who responded to all three.