Solovar is known in DC Comics for his position as the leader of Gorilla City, and we saw in The Flash Season 2 that Gorilla City is located on Earth-2.
Solovar will make his Flash debut in Episode 13 of Season 3 and stick around through Episode 14.
The Flash executive producer Andrew Kreisberg revealed at the recent TCA press tour (via TVLine) that one of the episodes will take place on Earth-2 while the other while take place on Earth-1.
Kreisberg did not specify which episode would take place on which Earth, but we can guess that the first half of the two-parter will probably be set on Earth-2.