[RPG] With Spirit Guardians up and someone casts Insect Plague on your group, do the locusts die before they bite you

As of the 2018 errata, if an enemy takes damage from your spirit guardians spell, your sanctuary spell ends

The description of the sanctuary spell (PHB, p. 272) originally said:

If the warded creature makes an attack or casts a spell that affects an enemy creature, this spell ends.

However, the 2018 PHB errata added "deals damage to another creature" to the list of conditions that end the sanctuary spell. The description of the sanctuary spell now reads (emphasis mine):

If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.

Even though the spirit guardians spell doesn't directly affect an enemy creature when cast, it does deal damage to them:

An affected creature's speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes 3d8 radiant damage (if you are good or neutral) or 3d8 necrotic damage (if you are evil). On a successful save, the creature takes half as much damage.

As a result, the sanctuary spell ends as soon as an enemy takes damage from a spirit guardians spell cast by the warded creature.

Even before the 2018 errata, the intent was that damaging an enemy would end the spell (though this was not reflected in the description of the spell). In an unofficial tweet in July 2016, rules designer Jeremy Crawford stated:

You first cast spirit guardians and then sanctuary. Does sanctuary end when spirit guardians deals damage to enemies?

RAW: Sanctuary isn't stopped if an already-cast spell deals damage. RAI: A damaging activity ends the spell.

RAI means errata or a future edition would possibly change RAW, right? Thanks for the answer

That's correct

Clearly, the 2018 PHB errata changed the spell so that it now works as intended.

Multiple of the same spell never stack. Player's Handbook, “Combining Magical Effects”:

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.

With three casters of spirit guardians you can cover more ground, but you will never get more damage or saves out of them by overlapping them.