It can be 6 units. The rule relating to the sides of the triangle is that you should be able to add two sides and get a length that is longer than the third side. You usually add the lower two numbers because it basically says that it will work for all sides but, if you're still in doubt you just have to compare the numbers.
For Example
3+5 is greater than 6
5+6 is greater than 3
6+3 is greater than 5.
All of these makes sense so 6 can be a possible answer.
