The numbers in the ND filters are powers of two (of light reduction), so stacking an ND8 with an ND2 gives an ND10. If you add the ND4, you get ND14, which is normally enough to photograph directly into the sun.

However, depending on the filters, they do not filter out infrared or ultraviolet, so the sun might still fry your sensor (or your eye, if you look into the viewfinder).

Filters specifically made for sun viewing will have their filtering capacity extending sufficiently into IR and UV ranges (unless they are cheapo fakes...).

To your question, the difference is that you will have a multitude of glass surfaces, that will deteriorate the picture quality. ND filters are often poorly coated, and having six surfaces of poorly coating results in quite some reflections, etc. still, this is probably not too big an issue, unless you are trying to do high-end photography, for billboards or large prints. For computer screen viewing, or 6x8 prints, that should not be an issue.