I don't know if this is philosophical or physics…but hopefully we don't have to go there:
Inspired by:
Does the Spirit Guardians spell stack with multiple casters?
How do you determine the 'most potent effect' for overlapping spells?
The effects of the same spell cast multiple times don't combine,
however.
So the question I have, is how do you clarify this with the fact that two wizards can cast a fireball (instantaneous) as a readied action at the same time? Do the rules clarify no two "instantaneous" things happen at the same instant, ever, in the whole world?
The only thing I can come up with, is that nothing in the game is supposed to happen at the same time.
This is not indicated for readied actions. So should you also resolve multiple readied actions with initiative rolls and d20 tie breakers?
Note: It seems this would also determine, for instance, a monster affected by an enchantment such that damage breaks the enchantment. I had ruled before that players can surround such a monster, each ready an attack, and each make one attack with advantage before the monster gets a chance. This approach adds realism and removes boring cleanup from the end of combats. However, if the readied actions can't happen at the same time, then it really is the first hit that would wake the monster up, and I am technically breaking rules with this.
Best Answer
Actions are not designed to happen at the same time, rather they happen in discrete chunks.
This is clear from the design of turn based games in general, and especially D&D. This is also apparent, as things that grant reactions usually specify specific timing, either before or after its trigger. Example of the Ready Action (PHB p. 193):
There are no clearly written rules for resolving multiple readied actions on the same trigger.
However, the easiest, and most straightforward way to resolve this situation is in initiative order for all those readied on the same trigger, and seems to best fit the spirit of the rules, and how they work in general.
An alternative to this, would be to resolve order based on dexterity bonus, however, this introduces additional ties, which should already be resolved by initiative order.
Your last example does let the monster "save" for each attack, but it's still a valid and powerful tactic.
Preventing the monster from responding in between each attack is still a powerful benefit, and may potentially finish it off before it gets another turn.
Being hit while asleep from the Sleep spell, they would wake up on the first strike and lose the unconscious status, and being hit by something that gives a save on damage, such as Tasha's Hideous Laughter, would give them a save for each hit regardless of if all actions happen simultaneously or not, as per the wording of the spells, as each hit is still a separate instance of the target taking damage. Note that victims of Tasha's Hideous Laughter and Sleep would still be prone until they get a turn to stand up.
Finally, the general rule is that things are resolved in initiative order.
There is no specific rule overriding that. Following D&D 5e's system of general rules applying unless a specific one overrides it, I'd conclude this is likely the RAI and RAW approach. See SRD p. 90: