Of course you want to fill in the gap with epoxy thickened with..... speed said Cabosil, I've used something like that (fine powdered silica.) I've also used milled fiberglass and balsa sanding dust.
Then lay down the 5", and follow up with the 3".
I generally just do the one layer and often it is no more than 2" wide. However, do it speed's way an you will indeed have a very strong wing that you don't have to worry about.