The name of S03E07 of Star Trek: Discovery is Unification III. The Unification word has some connection with the story, but what does the suffix III signify?
There have been two episodes in Star Trek: The Next Generation:
In these episodes, Spock travels to Romulus in order to reunite the Romulans and Vulcans as one people.
The synopsis of the episodes from the respective Memory Alpha articles linked above:
To the Federation's surprise, Ambassador Spock has traveled to Romulus. Fearing he has defected, they send Captain Picard and Data on a covert mission to determine why.
On Romulus, Picard and Data meet with Spock, who claims to be trying to reunite the Romulans and Vulcans. While Spock works to achieve his goal, powers within the Romulan government seek to pervert his mission into an invasion of the Federation.
So, the reunited Romulans and Vulcans are a result of what Spock did, therefore Unification III.