[RPG] Does a +1 shield allow me to add 3 to the Dex saves when using the second benefit of Shield Master

The language is thoroughly ambiguous. It can be interpreted in either of two ways:

• (you can use your reaction to take no damage) (if you succeed on the saving throw)

  If you succeed on the saving throw, you can use your reaction to take no damage.

• (you can use your reaction) (to take no damage if you succeed on the saving throw)

  You can use your reaction to set up a delayed trigger to take no damage on a successful saving throw.

The main question being whether "if you succeed on the saving throw" is part of the effect, or a further condition on the reaction.

There is plenty you can infer here. For example:

• If they intended "if you succeed on the saving throw" to be part of the condition, they could have grouped it with the other condition and come up with something much clearer: "If you [must make a Dexterity saving throw to take half damage], and you succeed on the saving throw, you can use your reaction to..."

  Since they didn't use this wording, clearly they must mean for the reaction to be used before the save is rolled...

On the other hand...

• There are other effects which explicitly state they must be used before a result is known/rolled/etc.

  Since they didn't explicitly state that you need to use your reaction before knowing the result of the save, clearly they must mean for the reaction to be used after the save is rolled...

Ultimately, the choice is your DM's. Or Jeremy Crawford's, if he gets around to providing a Word of God answer.

(insert rant about sloppy templating)

The spells from the Player's Handbook that target a single creature and trigger a Dexterity saving throw are:

• Acid Splash, as long as the caster doesn't choose to target 2 creatures.
• Chain Lightning, if there are no other targets within 30 feet of you.
• Disintegrate
• Hellish Rebuke
• Otiluke's Resilient Sphere
• Sacred Flame

And from the Elemental Evil Player's Companion:

• Immolation

None of the new spells from the Sword Coast Adventurer's Guide meet the required conditions.