Should I extend existing 12/3 circuits with 12/2 for wiring attic?
You can run 12-2 from the box to the other locations. If a double breaker one with a single handle or handle tied is installed you could use the red wire for a 2nd 120V circuit using the same ground and neutral. This is legal and called a multiwire branch circuit. This could provide up to 4 circuits if both "spares are 12-3. Capping the red is a fine way to do it if you only need the 2 circuits and want to run 12-2.