There is only one requirement for three sides in order to be considered possible in constructing a triangle. That requirement is
The sum of any of the two sides must be greater than the third side
11 + 15 = 26 > 21
11+ 21 = 33 > 15
15 + 21 = 16 > 11
Since all of the combination fulfill the requirement, yes we could create the triangle with those sides.