Do you really need 3 layers of fibreglass? If just a bit worn with gelcoat left you do not need to do that. If all the gel-coat is gone, it's a judgement call: are you down to the fibers or not?

What I've done on my nacra was to sand the bottom with coarse sandpaper to clean it up. It was then obvious the places that needed to be build up and the one just needing new gelcoat.

Near the transom, where a lot of material was removed by 20 years of use I used 4 layers of fiberglass/epoxy to rebuild the shape. I also used two layers in the back of the daggerboard well as it was showing signs of wear. In a couple of places I just put some epoxy with a roller to reseal some fiber properly.

I then re-sanded everything I added with the same coarse sandpaper to make a good clean rough surface and sprayed gelcoat on top of it. One pass with gel-coat thinned with acetone to get it to the point where the whole bottom is white, and then a second pass with gel-coat with wax add-on so it cures. And then a lot of sanding/buffing.