I'm not sure if you can use numeric examples here, since I asks you if X is twice as long as Y after adding the three meters. We cannot know if there is a relationship between X and 3, so I wouldn't go to the numeric example...
Let's use what we have for now. And that is:
x = 2y
y+2 = z.

Now we add 3 meters to every fence. Let's analyze each statement and see if it is true or not.
1. x = 2y. While this is correct before adding the 3 meters, it is not after adding them. If it were, you get that 2(y+3) = x +3, which means that 2y+6 = x + 3. Since we know that x = 2y, this means that 6 = 3, which is obvious nonsense.
2. y+2 = z. Let's add the 3 meters to y an z. What do we get? Well, y + 3 + 2 = z + 3. This formula is consistent with the initial one, that y+2=z, so we assume it is correct.
3. After adding the three meters, we need to check if x + 3 is greater than z + 3, which is the same as asking if x is greater than z. Well, let's use our initial two equations. x = 2y and y + 2 = z, this means that x/2 + 2 = z. Is x greater than z? Well, we can't be sure. If you consider x = 2 as in the Kaplan explanation, then this is not true. But if you consider x = 6, then it certainly is true.

So I'd go with:
I. not true
II. true
III. not necessarily true

Since you are looking for smth [b]that must be true[/b], then your answer will be II.

Thanks both of you

-------------------------------------


OUCH

How often is this kind of questions are in the GMAT?

Can I say that if it asks "must be true", I better random twice?