What is the dimension of the filters if the input image has only one channel?
It is better, in most cases, to convert Grayscale images to RGB images with equal corresponding values in the 3 channels.

In this case, your filter size will be NxNx3.

The reason this approach is preferred is, among other things, if some time in future, you decide to use a pre-trained CNN you would most definitely need an RGB image to use its weights.