You might want to check that you are allowed to use a programmable calculator.....
That aside, the CR-3 is cheaper and way more logical. It doesn't use a slide, and is based on cosines. The slide rule side is similar to the others, it is true, but it's the wind side that scores - it makes grid navigation a breeze.
In the picture, your true course is 130. Your grid course is shown opposite your longitude, in this case 090 degs opposite 40E.
You can do this a little bit with the CRP-5, but it goes all the way round on the CR-3.
Also, convergency.
180 nm on the outside scale, find the cosine of the latitude, in this case 54 degs N, departure is right opposite (the sine of 36 is the same as the cosine of 54).
Busted!
I do indeed work for an ATO .