You could try 1 of each. 1GT 1JD and 1O. Make sure that the O is larger than the other two by a little, and that the GT is larger than the JD. If you don't have "pairs" then they'll have less reason to harass the Oscar. I'd have a 4" Oscar, 2.5" GT and a 2" Dempsey. Give the Dempsey a cave in the corner and he should grow up peacefully so long as nobody takes his cave. Just remember, that even though it will take a while for the GT and JD to reach their full size, those three could be quite a handful in a 75 after everyone is over 8".
None of the mixes we give you will be foolproof, but this gives you the opportunity to keep three of the four fish you had mentioned, with less chance of a death. Just make sure that the GT is larger than the JD. Female Jd may be best.