In order to keep nitrAtes at or below 20 PPM, you'll be looking at 4 large water changes (50%+) per week, minimum, once those fish are all full grown. There is evidence showing physiological damage in fish kept above 21 PPM nitrAtes over the long-term, and that damage substantially increases once levels approach and exceed 40 PPM.
Of course, having all 3 oscars survive together into adulthood is all contingent upon them managing to not kill each other first. Keeping 3 oscars together typically will result in two becoming buddies---possibly even pairing and producing offspring---and teaming up against the 'third wheel.'
Getting one moved to the future 75 gal. you mentioned before things get too out of hand would be a good idea. As for the other two...if they've paired (i.e. as male/female), then you may stand a chance at long term success, but there's no guarantees. If they're both female, the same applies. If they're both male, forget about it. An 80 gal. tank really isn't a lot of space for oscars, even if you do have a decent footprint on there---what are the dimensions of the tank?
BV