DO not treat until absolutely sure its whitespot
and 3 oscars in the tank is a death sentance for aatleast one of them i'm afraid never keep oscars in odd numbers its either 1, 2 if you get a decent tempered couple of fish or 4/6 but that needs huge tank.
forgot to add if tank water is of good condition the O will heal pretty quickly with no meds dosed but melafix wont hurt and will help stop infection but your water stats seem good enough to let it heal naturally