They are both neutered/spayed.
They've been together now 3 weeks. It seems to be a fight a week
Although their first fight was after 6 days, hten 7 days and i htink this one was 9 days.... so progress right?
If they have only been together for 3 weeks it is likely that they are not properly bonded. Go back to basics, small neutral space and start as if you are bonding and then VERY slowly increase their space.
What size space is the biggest space that have they been in recently?