I have a +3 bow with mighty 3 also

A missile weapon with the Mighty ability lets you add part or all of your positive strength modifier to the damage you deal.

If your bow is Mighty +3, that means you can deal normal damage (if your strength is 3-11), normal damage +1 (strength 12-13), normal damage +2 (strength 14-15), or normal damage +3 (strength 16+).