Yes in a year or so , we are planning on getting a 200 - 400 G tank. My oscars are about 5 inches. Two tiger and one albino ( and its so mean).I would definitely add a canister. I'm not familiar enough to advise you on a brand for the canister though, sorry...
And I hope you plan on upgrading the tank. 3 Oscars!? how large are they now?
How often are you doing water changes?