The potter probably doesn't want to apply the same glaze multiple times, in which case she has 15 choices for the first layer, 14 for the second, and 13 for the third, giving
15 * 14 * 13 = 2730
possible test vessels.
On the other hand, it's possible that the potter is fine with re-using the first glaze in the third layer. In that case, whichever glaze is used first is not counted among the options for the second layer, but it comes back for the third layer, so that there would be
15 * 14 * 14 = 2940
test vessels.