There does not seem to be an issue with the dimensions, but rather with the condition that you are using. Try the following:
if norm(vecnorm(b3-c2)) < 1e-5
The reason why you need to do this is because all the calculations made by "fft" are subject to numerical errors. So, when you are checking whether "b3" is equal to "c2", you should not expect all the decimal digits to match since both variables are matrices of doubles.