Does the 3rd part of Shield Master continue working against multiple, same-source effects?

Question

If you use the 3rd part of the Shield Master Feat;

If you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you can use your reaction to take no damage if you succeed on the saving throw, interposing your shield between yourself and the source of the effect.

If another Fireball is dropped in the same spot, during the same round, but later in the Initiative Order, does the shield still help you? Or if a wagon full of explosives goes off, the DM wants it to work for only one bomb in the crate, not all of them. Many Reactions, once activated, keep working for the rest of the round. Since the Shield is still in the same protective place and the source is still the same, does it still provide the same protection?

Let me again be very clear. Same round. Same locations for the character, shield and the source of the effect. Does Shield Master continue working?

Answer 1

No, you only get one reaction per round.

Unfortunately, once you've used your reaction on one effect, you can't use it on another effect the same round.

Reactions are bounded as follows:

When you take a reaction, you can't take another one until the start of your next turn.

Shield Master also specifically says (my emphasis):

If you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you can use your reaction to take no damage if you succeed on the saving throw, interposing your shield between yourself and the source of the effect.

An effect is singular, not plural. This coincides with the single reaction per round. You can use this against a singular effect and it doesn't last until the start or end of your next turn like some other in-game mechanics.

Answer 2

Using this feature requires and uses up your reaction for that round (you have just 1) so you can activate the effect just once. Other requirement for activating the effect is succeeding in Dex saving throw.

So if you need to make another damage-avoidance Dex saving throw in the same round, after using this feature when you succeeded in the previous saving throw, then you can't use this, and receive the usual half damage.

Also note that even if you used your reaction for something else, you can't use this feature until your next turn comes.

Answer 3

No, but...

No, as other answers have stated, you only get one reaction per round. The third effect of Shield Master requires you to expend your singular reaction in order to negate the damage you would take.

Reactions:

When you take a reaction, you can't take another one until the start of your next turn.

and Shield Master:

If you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you can use your reaction to take no damage if you succeed on the saving throw, interposing your shield between yourself and the source of the effect.

I will throw in two "buts" here though:

First, it should be noted that the second effect of Shield Master would still apply, as it requires no reaction or anything else on your part; it's just a passive effect:

If you aren't incapacitated, you can add your shield's AC bonus to any Dexterity save made against a spell or other effect that affects only you.

Second, you mention a cart full of explosives going off. In such a scenario, you typically would be able to use your reaction to negate the entire cart's explosion, rather than only a single stick of dynamite's explosion from inside the cart. This should be handled similar to a Necklace of Fireballs. A single bead goes off, it's one fireball explosion. If a user throws 3 beads together, it's not three explosions, it's one more powerful explosion. I don't know many DMs that would rule one cart's explosives going off as 50 separate events ("the first box in the cart explodes, would anyone like to do anything?...now the second, would anyone like to do anything?...now the third..."); it'd just be one event ("the cart explodes")