If the author considers "sight, smell, and hearing" as one set, he or she may use a singular verb. "Sight, smell, and hearing" could be considered as the "one set of three major senses" for example. In other words, he is speaking about a situation in which "sight, smell, and hearing" are not understood as three individual senses but as one set or package of three senses.
What do you do when water, orange juice, and milk is not enough?
Here the author, me, is not considering water, orange juice and milk as three separate liquids but as one set of three liquids that together is not enough.