I am really loving season 3! I love the characters old and new. When do you think Burton, Sirtis, and Spiner will join? I am really excited to see Geordi!
I don't know. But if I had to take a stab at a guess, I'd say Episodes 5-6. Only a guess. At maximum, I think everyone will be there by the end of Episode 8. It sounds like Episodes 9-10 are going to be the Third Act, so it would be pretty late in the story to re-introduce them then.
We saw Troi yelling at Riker in one of the season trailers, but after their nice talk in Ep4, it seems a little out of place now.
If Lore does appear in ST:Picard, life for Picard could be rather, interesting, to say the least. Especially since Lore 'freed' some of the Borg and became their father, I think Lore would have a very difficult time being the last of his kind along with discovering that the X-Borg no longer need a ,'motherly figure', in their lives.
Troi and Geordi show up by at least episode 6. One of the photos of Geordi on the Paramount Press site is tagged as being from episode 6. Terry Matalas says in this interview that Troi starting filming in person for episode 6 and that the last five episodes are Troi heavy episodes. https://collider.com/star-trek-picard-season-3-episode-4-spoiler-interview-showrunner-terry-matalas/