The probability that all three of them will buy the same donut is zero. Donut shops usually respect property laws and don't sell things they've already sold to other customers.
Assuming that the question was intended to ask about the probability that all three of them will buy the same type of donut, the question arises whether the sentence "You may assume that Nicole, Breda and Amanda are equally likely to buy any of the donuts available." is intended as written, or is also intended to refer to types of donuts.
If it's intended to refer to types of donuts, the answer is $1$ in $25$: There are $5^3$ combinations for the five types of donuts, they're all equally likely, and in $5$ of them they all buy the same type of donut. Since we wouldn't need to know how many of each type there were left to answer this, this is presumably not how the question was intended.
So assume that the question was intended to ask about the probability that all three of them will buy the same type of donut, given that they are equally likely to buy any of the donuts available. In that case, there are $\binom{15}3$ combinations of choices, all of which are equally likely, and there are again only $5$ combinations in which they all buy the same type of donut, so the probability in this case would be
$$\frac5{\binom{15}3}=\frac5{455}=\frac1{91}\;.$$