Jags are territorial so they will chase or attack any fish in their space. A new fish will really get it. Green Terrors are not as territorial but if the Jag was chasing the RD into its area, the GT would go after it. It would be hard for all three of them to live peacefully together in a 125 without being introduced together at very small sizes. Even then as a RD matures it becomes more aggressive and will attack its tankmates. Many people including myself have to keep their RDs alone because of their high aggression. If you try to introduce another RD, do it after a water change and after rearranging the tank. This will help but in the end, you still may end up with dead fish one day if the Jag or Red Devil decides that they no longer want to play nice. It's not impossible to keep them together though. Tank space and individual temperment play major roles. I wish you luck the next time around.