Multiple, independent Borg Collectives are an old characteristic of the Borg
The Borg are introduced as a huge, completely interconnected hive mind, which is one characteristic that makes them unstoppable.
However, this characteristic leads, out of universe, to two problems:
- To be a proper, enjoyable villain, the Borg need to be "defeatable" at the end of the day, otherwise TNG Season 4 and onwards would have been very boring.
- Given range limits in communication, how can the Borg maintain interconnectivity with the whole hive mind at all times? And if they possess the technology for long-range communications, what would happen when that fails?
So the plot device they found is that it is possible to "split" the hive network, that disconnected parts of the network can act autonomously and can (and strive to) reconnect. And to add to that, they have added the idea that our individuality is something that eventually is dangerous to the Hive Mind.
They have used these plot devices multiple times:
- "I, Borg" in TNG deals with a Borg Drone disconnected from the Hive Mind
- "Descent" deals with a whole collective disconnected from the Hive Mind as a result from what happened in "I, Borg"
- "Unity" in Voyager marks the first appearance of the Borg in Voy and also shows a separated Collective
... and many more.
The Borg Queen in Season 3 herself is on her own
Now that we have established that disconnected collectives are a thing in Star Trek: The Borg Queen we see in Season 3 is clearly not connected to a huge hive mind and assumes it doesn't exist. Part of her bitterness and anger is (other than being part of her evil personality) driven by the fact that her collective was dying from Janeway's virus and she even had to kill her remaining few borg drones to sustain herself.
Long story short: The universe might be full of other Borg, including Jurati, but this Borg Queen in Season 3 is not connected to them and wouldn't know.