I would wait to introduce your jag until it is about 5"..or until your sure your oscars can't swallow it. Small jags are fairly shy in my experience until they reach the 5-6" stage. They also seem to grow slower than oscars. I have raised jags with green terrors, jack dempsies, oscars, midas cichlids, and once a TSN when small and all about the same size..under 5 inches. The young jags were often shy, pushed around and bullied at this stage. Grew a little more slowly it seemed than the above mentioned fishes(except the green terrors) and spent lots of time hiding.
But at the 5-6" stage things normally change. Jags tend to get quite a bit more aggressive and territorial. Assert dominance over other tank mates. I have only had success keeping them with oscars, midas, and 1 dempsey(who was constanly bullied and spent most time in seclusion) after the 6" mark. They also tend to cohabitate with just certain individual fish, while mercilessly persecuting others. The current jag I own was grown out with 2 oscars and a TSN, all aquired at about 2 inches. After reaching 5" he accepted one of the oscars. Swimming together and hanging out. But it almost killed the other. Had to be seperated. My jag and his oscar buddy are now 12-13" living in a 90 g. show tank. All is fine as long as the oscar stays to it's quarter of the tank. If the O encroaches upon the jags domain, it is liplocked and driven back to it's area.
I would not recommend keeping jags with bottom dwelling fish. Too easy a target and they seem to pay no attention to territorial boundaries. My TSN was almost killed by my current jag after growing up together and being twice the size of the jag when the aggression started.
When you do introduce the fish, just keep a close eye on them. Provide some cover for hiding. Be ready to move some of the fish depending on behavior and aggression. 2 oscars could pair up and bully your jag. Jag might not accept either of your oscars after getting acclimated. All 3 could be like peas in a pod. Every case is different depending on individual fish personalities.
I'm guessing your tank is 6' long. The one thing I will almost garuantee is that after hitting the 8" or so mark, your jag will claim at least half of it.
Have fun and good luck