Depends on what size the fish are now. It also depends on the aggressive nature of jack and the green terror. In my opinion i would go with a slightly larger oscar as there not one for batttling it out with other cichlids of the same size. Every oscar I had eventually died in an aggresive cichlid tank. However, most of the jack dempsey's i have had have been more docile than say the REd Devils or manguenas(sorry about spelling). I have read green terrors as being very aggressive, but they too never seem to be tough enough to survive in any of my agrressive cichlid tank. SO the three might work out in a big tank, then again they may not. If you start them out smaller they would be more use to their fellow tank mates when they got bigger.
Jack dempsy gets to about 9 to 10" , green terrors 7 to 8" , oscars get real big-13" to 14" ..I've seen guys keep 2 or 3 oscars in a 90 gallon, but the water always looked cloudy and dirty. You could always buy a 75 gallon and wait it out, its going to take you quite awhile for them to get to their maximum size. Then buy a bigger tank later and do something else with the 75.