Please note that the 3" measurement you see in that diagram is not the width of the base, it's the spacing between the two outermost screw hole centers. Even then it's not really a vital dimension. These screw bases come with multiple different screw hole positions to accommodate many different kinds of windows, including your own.
As for your arm not being straight, I'm not sure what you mean. Looking at your photo it appears straight to me. Can you clarify what you mean?