Assuming the target is able to be hit and does not have immunity to force damage, would firing all three missiles at a target with 0 hit points inflict three guaranteed death saves ergo death?
Magic Missile: You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range. A dart deals 1d4 + 1 force damage to its target. The darts all strike simultaneously, and you can direct them to hit one creature or several.
Let's start with when to make Death Saving Throws (PHB, 197)
If you take any damage while you have 0 hit points, you suffer a death saving throw failure.
So any time something gives you damage while at 0 hit points, you suffer a saving throw failure.
As quoted above in the question, the language for Magic Missile is separate per missile. Each missile strikes simultaneously, but the damage from each is separate. If we are separating damage, we are separating those Saving Throw failures.
The Concentration function is similar in that it triggers off of damage (PHB, 203):
Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw.
However, it also goes on to state:
If you take damage from multiple sources, such as an arrow and a dragon’s breath, you make a separate saving throw for each source of damage.
Jeremy Crawford specifically ruled on Concentration saves and Magic Missile requiring each missile to force a new save.
Roll for each missile
That leaves us the real question here as to whether or not Magic Missiles are separate sources of damage, or if the missiles hitting concurrently represent a single taking of damage.
Given that Crawford seems to believe that magic missiles are multiple sources that require a roll per source, it seems you can extrapolate that the missiles are also giving damage separately (while hitting concurrently) and thus forcing death save failures for each magic missile. The concurrent nature of the strike doesn't override the multiple deliveries of damage from separate sources.
Caveat Emptor
If you are a DM and planning to target a PC like this, you must beware of hard feelings at the table. Typically death saves are a PC thing, and if you are going to remove that by going after unconscious players then the possibility of hurt feelings is real. Most monsters are more concerned about taking out other creatures who are alive rather than double-tapping any downed creatures. If there was a real reason by the NPC to do this, then that's different - but if you're just going after unconscious players because they're easy targets to completely kill you may suffer backlash from your table.
We know that each Magic Missile dart strikes simultaneously (from the spell description), and we know that damage is rolled only once for all of them. But, we also know that each dart is a separate source of damage. So, our question really boils down to this: Does a death saving throw failure happen for each time that a 0 hp creature is dealt damage, or for each source that deals damage?
While there's isn't a direct official ruling on how multiple Magic Missile darts affects death saving throw failures that I could find, there is a ruling on a similar question: If you are concentrating on a spell, and get hit by three Magic Missile darts, do you need to roll a concentration saving throw for each of them? Per a Jeremy Crawford tweet,
Concentration: "You make a separate saving throw for each source of damage" (PH, 203). Roll for each missile.
The rule referenced there (I'll quote from the online Basic Rules):
Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon’s breath, you make a separate saving throw for each source of damage.
So concentration saving throws specifically say to look for each source of damage, which is the basis for Jeremy's tweeted ruling. Now that I've gone on that tangent with concentration, let's compare that wording to the wording on taking damage while at 0 hp:
If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead.
There are two ways one can look at this:
But unless we get further official guidance, we don't really know which way it is "supposed" to be, and I've already analyzed the text much more than was probably intended.
As with all the rules, reading and interpreting them is up to your DM. If this scenario is likely to come up in your games, it's best to work out how your DM will rule this case ahead of time.
And if you're the DM, just go with whichever interpretation you think is most correct or will lead to the most fun gameplay, and ensure your players know how your game will work.
Based on the wording that the darts strike simultaneously, you would only suffer one failure. You suffer each dart's damage at the same time
...A dart deals 1d4 + 1 force damage to its target. The darts all strike simultaneously, and you can direct them to hit one creature or several.
At higher levels: When you cast this spell using a spell slot of 2nd level or higher, the spell creates one more dart for each slot above 1st.
Similarly, if you took damage from buckshot, you wouldn't suffer a death failure for each individual pellet.
Darts are defined as dealing 1d4 + 1. You get more darts at higher levels. Every dart can strike a separate target or the same target. The reason the darts are defined separately is because they can hit multiple targets and because of the nature of the "At higher levels" clause, which increases the number of darts.
If you take any damage while you have 0 hit points, you suffer a death saving throw failure.
So you're not taking three instances of 1d4 + 1, you're taking one instance of 3d4 + 3. A general rule of thumb is there is 1 damage instance per attack (or per spell cast). An attack is often defined as anything involving an attack roll in the books, but this rule of thumb can be extended (by way of common sense) to include effects involving saving throws, singular spell casts and actions if there was no save or attack roll.
So if you make an attack roll and deal damage, you've done one attack and therefore one death failure. Multi-attack's and Extra Attack's damage is therefore separated by each attack roll.
If your target rolled one saving throw and took damage, then they have one death failure.
Else if (in the case of Magic Missiles), you've cast one spell and they took damage, so there is one death failure. Or with Witchbolt, you've used an action to cause damage automatically on your turn. This witchbolt example is mostly a common sense example in order to help understand the logic here.
