wouldnt this second optimizie "override" the first one?
Partly, yes. The "-a 3" fast optimization only moves files to defragment them and to fill up gaps. It leaves the other files where they are, so most of your disk remains the same.
when is the best time?
Whenever you like. The "-a 3" optimization is a daily optimization, to clean up fragmentation and to fill up gaps created during that day.
and when should i do "-a 7" again?
A good time would be after installing or deleting a lot of programs, yes. Or once per month, or something like that.