Third one is right.
Two sides and the angle between them.
The cosine trigonometric function is defined as:
cos(x) = adjacent/hypotenuse
I.e. the cosine of an angle (within a right-angle triangle) is equal to the ratio of the adjacent side (to the angle) to the hypotenuse, where the angle is one the two that is not the right-angle.
If you illustrate this, you can observe that the components of the cosine on a diagram are always the two sides that create the angle and therefore the angle will naturally be between these two sides.
