The enchantment condition itself is quite limited in what it does:
- An enchanted creature can not attack the enchantor or attack the enchantress.
With harmful abilities or magical effects.
- The sorcerer has an advantage in any ability to interact socially.
with the creature.
The action of the charm of the succubi is more specific (emphasis mine):
Charm. A humanoid that the demon can see within 30 feet must succeed on a DC 15 Wisdom saving throw or be magically enchanted for 1 day. The enchanted target obeys the verbal or telepathic commands of the devil. If the target suffers some damage or receives a suicide command, it can repeat the saving roll, ending the effect in a success. If the target is saved successfully against the effect, or if the effect ends, the target is immune to this demon's charm for the next 24 hours.
The devil can only have one enchanted goal at a time. If you love another, the effect on the previous goal ends.
If a succubus bewitches a creature and fails the salvation of Wisdom, it must obey the verbal or telepathic commands of the succubus. The only exceptions are if he suffers any direct damage or receives a suicide command (such as throwing himself into the lava), in which case he can repeat the salvation. If the succubus commands an enchanted target to attack an ally, the enchanted target follows that order.
The succubus (C) can Enchant B, and then (assuming that B fails to save) the B command to attack A. So far, all right.
Then, we observe the Fey Step function of the eladrin (MToF, page 62):
As an additional action, you can magically teleport up to 30 feet to an unoccupied space you can see. Once you use this feature, you will not be able to do it again until a short or long break is over.
When you reach the 3rd level, your Paso de Fey gains an additional effect based on your season; If the effect requires a saving throw, the DC is equal to 8 + your skill bonus + your Charisma modifier:
Autumn. Immediately after using your Fey Pass, up to two creatures of your choice that you can see within 10 feet of you must succeed in a Wisdom salvation throw or be charmed by you for 1 minute, or until you or your Comrades hurt that.
Unlike the succubus, the fall eladrin Fey Step simply imposes the enchantment condition in a failed save, with no additional effects. Nothing about the condition forces the enchanted target to obey the orders of the eladrin in general. The enchanted target simply can not harm the eladrin directly, and the eladrin has an advantage in the controls of social interaction against the enchanted target (for example, control of persuasion, intimidation, deception or another type of charisma not associated with specific abilities).
The eladrin (A) can use Fey Step to teleport 10 feet from the succubus (C) and then try to charm it; If the succubus fails in the save, the eladrin loves it. This prevents the succubus from attacking the eladrin directly.
However, nothing about this situation alone affects the previous command of the succubi to B, who is still enchanted with the succubi, so if nothing else is done, B will continue to attack A (the eladrin) in the turn of A. A could try to convince C (the succubus) to tell B to stop attacking, but there is no guarantee of success.
Since C is an enemy monster, the Charisma check of A (with advantage) would be made against a fixed DC; since both parties were previously in combat, it would probably still be difficult enough to convince the succubi to stop the attack. Of course, the DM may decide that the enchantment condition provides a magical stimulus for the enchanted target to meet the demands of the most attractive enchantors, and that is why they may decide to set a less extreme CD for the check.