The triangle inequality theorem states that the sum of any two sides 
of a triangle is always greater than the third side.

There are four groups of three of the numbers 3,6,10,5. 

3,6,10  <-- These cannot be the three sides of a triangle
            because 3+6 < 10 and the sum of any two sides must
            be greater than the third side. 

3,6,5   <--These can be the three sides of a triangle because
           3+6 > 5, 3+5 > 6, and 6+5 > 3 

3,10,5  <-- These cannot be the three sides of a triangle
            because 3+5 < 10 and the sum of any two sides must
            be greater than the third side. 

6,10,5  <--These can be the three sides of a triangle because
           6+10 > 16, 6+5 > 11, and 10+5 > 6

Edwin