In order for {3} to be a subset of {1, 2, 4, 5, 6} it must be true that if
you take any element from the set {3}, that element must also be one of 
the 5 elements of {1,2,4.5.6}.  There is only one element in {3} and that
is 3.  So the question becomes this: 

Is 3 listed among the elements of {1, 2, 4, 5, 6} which are 1, 2, 4, 5, and 6?

To answer that, look and see if you see 3 among these five elements:
 1, 2, 4, 5, and 6.  Do you see 3 listed among 1, 2, 4, 5 and 6?

If your answer is yes, then {3} is a subset of {1, 2, 4, 5, 6}

If your answer is no, then {3} is not a subset of {1, 2, 4, 5, 6}

Which will it be?  Think real hard.

Edwin