Let's say Garry has 15 max HP and his current HP is 3.
Scenario 1: An enemy with 2 attacks a turn hits twice, dealing 12 damage the first time and 6 damage with the second attack; technically, the enemy did deal 18 damage, which is equal to Garry's max HP. Is Garry instantly dead, or is he still alive because the damage wasn't all from a single hit but 2 hits?
Scenario 2: Garry walks into a room with 3 archers who readied attacks to shoot him. First archer deals 9 damage, second one deals 9 more, and third one deals 3. Is Garry dead since it all hit him at once, or not since it was all from different people and attacks?
Best Answer
Each attack is a separate source of damage
When you have the Extra Attack feature, each attack that results from it is a complete attack that is resolved distinctly from the other. The PHB outlines how each attack resolves:
Thus, after each attack you will roll and determine damage. If any one of those attacks meets the criteria for Massive Damage, they target will be killed instantly.
Notice that you consult the Massive Damage rule whenever you take damage. And you take damage at the end of each attack. So, since each attack is resolved separately, the damage from multiple attacks is not added together for this purpose.
Scenario 1 - not dead
Garry is not dead, since neither individual attack did enough damage to count as Massive Damage.
Garry will have one or two failed death saving throws depending on if the attacker was within 5 feet or not (due to the critical hit rule).
Scenario 2 - not dead
If multiple creatures Ready attacks, each attack is still resolved separately. So this does not change anything with regards to Massive Damage. Since each attack did less than 18 damage, none of them kill Garry outright.
However, Garry does now have two failed death saving throws because they have been hit by two attacks while at 0 HP.