From the looks of it it may be that the 2 have paired up and started to bully the third. So most definitely a priority to get the third into a different tank as to not be killed by the pair. Honestly tho you should probably get a 55 gallon tank for them, they can be extremely territorial fish and love to have plenty of caves and sand. Also when it comes to African cichlids it's best to over crowd then under crowd. If it's under crowded they will lay claim to areas of the tank as their own and will mercilessly beat other fish away from their spot while if it's over crowded nobody can claim any one spot in the tank as there are just too many fish to chase off. Granted this only works in a bigger tank.