Swarms are (implicitly) treated as singular creatures
All of the information we need to mechanically handle a given monster is provided in its stat block:
A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster.
Stat blocks will also explicitly provide special saving throw information when it is present.
Next, we can look in the Sage Advice Compendium on page 16 for a confirmation that swarms are indeed groups (in an answer about the conjure animals spell):
... swarms are groups of creatures.
However, given that swarms are listed as their own stat blocks in the Monster Manual, we must consider that those stat blocks are treated the same as any other, and would therefore make saves, attacks, etc. in a similar fashion to obviously singular entities. A swarm's stat block represents multiple creatures but is treated as a single creature.
Importantly, note that the stat block doesn't detail how many creatures are in a given swarm, only that the attacks can differ when the swarm drops below half its starting HP (as detailed in the attacks for a given swarm). We could say that the HP represents how many creatures are in a swarm, but that would only be an interpretation, not a rule, and shouldn't change how we handle rolls.
In short: While Swarms clearly are groups of creatures, they are listed as stat blocks in the Monster Manual, et al., so treat them the same as any other singular creature with a stat block.
In your specific scenario, we go with option 1:
Considered as a single creature [here, would be a single CON save]