3, 2, and 1 can be the sides of a triangle because 3 plus 2 is greater than 1.

There are two requirements which 3 numbers must meet in order to
be the sides of a triangle.  

1. smallest side + largest side > middle side
2. smallest side + middle side > largest side

Checking 1:  1 + 3 > 2
                 4 > 2

True, so it does meet the first requirement.

Checking 2:  1 + 2 > 3
                 3 > 3

False, it does not meet the second requirement,
so the three 3, 2, and 1 cannot be the sides of 
a triangle. 

Edwin