Kim
Congratulations on getting your new 125 gal. Tank. Before adding a lot of fish be certain your tank has completely cycled. As for keeping multiple Oscars in your tank, you do have plenty of room, but you would be surprised how small your big tank will look with 3 full grown Oscars swimming in it. Not to mention the headaches involved trying to handle the aggression issues. If you are set on more Convicts, might think of keeping only females. Cons are prolific breeders and become a big problem when guarding a nest.
Before taking the big fish plunge, research the following fish: Firemouth, Electic Blue Acara, Salvini Cichlid (female) Electric Blue Jack Dempsey, maybe some of the larger Earth Eaters. Also look into mixing in some large Tetras such as Buenos Aires, Red Eyes, Diamonds and Black Skirts.