In my experience, yes, they'll be okay. Should any of the three become aggressive, you will want to have decor that can break the line of sight to disperse the aggression. For the new fish, you'll definitely want to get the same, or near the same, size of the current fish. I'd also recommend doing a thorough quarantine for the new incoming fish to be sure you have no issues when adding to the main tank.
Good luck and take care!
Phil