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Not seeing this 3", 1/2" to 3/4" deep scuff it sounds like a 3" gash with a nominal width much less than the depth? It's difficult to advise on the Marine Tex without knowing.

The gelcoat repair kits are basically for surface repair/restoration of chips, nicks, gouges and not filling.

Marine Tex and other epoxies with added fillers are quick and simple to use for deeper and narrow non structural repairs but you do end up with a rather solid plug with less flexibility that the surrounding structure.

If the area is wide enough to make the repair using some small pieces of resin soaked fiberglass in several applications you will end up with a close to structural repair that can then be gelcoat finished.