According to the information currently available, it appears the answer is no. The developers have not stated whether the Game Pass version players can participate in Immortal Empires.
Is Immortal Empires included in Game Pass? You also need to own the DLC on those platforms since the game cannot keep track of if you own any other versions of it in Steam and Epic Games Store. It was confirmed in the Total War: Warhammer 3 FAQ that you definitely need the previous two games on the same platform. Do you need to own Total War: Warhammer 1 and 2 on the same platform? It's essentially been redubbed the Immortal Empires for Total War: Warhammer 3. In Total War: Warhammer 2, this feature was called Mortal Empires.
To play with the DLC factions from those games, you need both games and the DLC. You will also need to own them on the same platform (Steam, Epic Games, etc.) Immortal Empires mode contains a new map where all of the previous factions can fight each other. Yes, Total War: Warhammer 1 and 2 are required to play the Immortal Empires. A separate campaign map called Immortal Empires requires both older games. Players won't have access to all of the features because the Main Campaign and Immortal Empires are completely different.
Players do not need to own any older games to play Total War: Warhammer 3's main campaign.