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 3 to be 115 or more? Why or why not?

The definition for "unusual" varies, but it usually indicates data lying roughly 1.96 standard deviations away from the mean (i.e. outer 5% of data). The corresponding value for z=1.96 is

100 + 8.66 x 1.96 = 116.97

115 lies within the middle 95%, so it would not be deemed "unusual."