Answer:
The correct option is 3.
Step-by-step explanation:
The difference of squares is defined as

In option 1,

This expression is a perfect square, therefore this is not the difference of squares. Option 1 is incorrect.
In option 2,

This expression is a perfect square, therefore this is not the difference of squares. Option 2 is incorrect.
In option 3,

This expression is the difference of squares., therefore Option 3 is correct.
In option 4,

This expression is a perfect square, therefore this is not the difference of squares. Option 4 is incorrect.