You cant keep 3 oscars in a 90, a 90 has the same foot print as a 75 but is taller which offers no benefit to large territorial fish. IMO a 55 is too small for a full grown adult oscar with a 75 being the minimum size i would consider placing one into, for two you would need a 120.
Oscars arent fussy about water as long as its clean, in dirty water oscars are very prone to hole in the head disease. A 40% water change or two smaller 20% water changes each week is recomended to keep the nitrates down to acceptable levels.