Assume that a population is normally distributed with a mean of 100 and a standard deviation of 15, would it be unusual for the mean of a sample of...

Your question is not very clear, I would recommend you
define what exactly you mean by "unusual." Here is my response to what I hope you want
to know.

The mean of the population is 100 and it is
normally distributed with a standard deviation of 15.

This
means that approximately 68.2% of the population has a value that lies between 85 and
115.

As normally distributed values have an equal
probability of being greater or lesser than the mean, approximately 15.9% of the
population has a value greater than 115.

If by unusual, you
mean a probability less than 15.9% it would be unusual for the mean of a sample of 3 to
be 115 or more.