This is the scenario:
Three creatures A, B, C are facing an enemy X, who is within 5'of all three of them. All have their reactions available. It comes to X's turn and X moves 5' away from the line of A, B and C provoking an opportunity attack from each of them as below:
- - - - - - |A| | | |A| | | - - - - - - |B|X| | → |B| |X| - - - - - - |C| | | |C| | | - - - - - -
It is ascertained that the order of these attacks comes in A, B, C order (see this question for answers about how this might be done).
A has not cast a non-cantrip spell this round and has the feat War Caster (PHB p.170) which allows the following:
When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.
A casts Hold Person (PHB p.251) using their feat, X fails its Wisdom save and is paralysed. As the following is true of an opportunity attack (PHB p.195):
The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.
then X never actually attempts to move out of the reach of B or C.
Do B and C get their opportunity attacks?
Are they cancelled as a result of the reaction taken by A or were all three triggered regardless of each other's subsequent interrupting reactions?
I would like answers supported by RAW, sage advice or the like as much as possible please.
I would like to remind you that it will be more problematic to argue that all opportunity attacks occur simultaneously as it is quite possible to have mutually incompatible reactions taken (two Polymorph spells cast simultaneously into different creatures for instance).
@RothbardWasRight uses a phrase that states the crux of the issue for me: "This is simply a special case of initiative." If it is then it is a compelling argument that B and C get no OAs.