Are you worried about the 3" not being enough thickness to withstand a solid hit? 3" is a huge thickness. It shouldn't easily crack.
I don't know where your crack was or how big it is, but unless you fill in the hollow section with structural material, the main strength that you are getting is from the glass mat in the keel already. Filled material will impart some toughness, but really not a significant amount of strength compared to an unfilled resin. You need load bearing material, such as continuous mat or roving, instead of chopped fiber. Yes chopped fiber will give you some increase in strength, but no where near what it would be for continuous fiber.
You could lay-up cloth around the inside of the hollow area, but you really need to ensure a good bond, else the interface, between the new and old plies.
I still think that a structural foam, shaped to the internal dimensions is the way to go. If you want to add additional strength, abrade the inside of the hollow keel, wrap some glass glass cloth around around the foam saturated with epoxy
, and shove it into the hollow area. The plies will bond the old keel to the foam plug and create a water tight seal and fill in the hollow and stiffen it.