I agree about a 300 gal.
I'm staring at my 180 at the moment, and trying to imagine 3 adult oscars in it, and to me it seems a bit wimpy.
Besides the lack of actually space, water quality and how much water would need to be changed in a tank that small, needed to keep them healthy might be more time and work than one might be willing to put in.
Since oscars are prone to hole in the head and other diseases related to stress (from high nitrates, cramped conditions, etc) in a 180, no matter how much filtration is provided, an almost daily or minimum every other day 40-50% water change might be required to keep them from being scarred up as they reach adulthood. The larger the tank, the better.
My minimal rule of thumb is 10 gallons per inch of adult cichlid.
So 3 oscars 12-14" long, add the height 6", and add the width, @3", is "minimum" 230 gallons.