I have to make a wedding cake and I'm doing the tiers with 3 layers, which end up being 6" tall. All the serving charts I find are for 4" tall cakes. Any idea about number of servings for 6" tall cakes?

4" tall and 6" tall are basically the same because it is based on the footprint not the height -- also they won't fit easily onto standard cake plates -- however if you plate at the 3" mark you can increase your servings but you have to cut each serving a bit bigger than the standard size to make up for the shorter piece

yes, you are not increasing your number of servings just increasing the size of the serving.  so from a normal 4" high serving to a 6" high serving is now 50% larger.  If your per serving was $4, it would now be $6 for that larger slice.

I've done a 6" cake with a separator plate at 3" which doubled the number of servings (well, depending if you cut 1 or 2 inch). But it's at least 1.5x the servings.