They watched all of Season 3 because they needed to determine what they have to work with. They likely all (rewatched) Season 2 with it.
Like every (non-mega) house they've done, I expect 8-12 scenes. That's it. And I expect because they didn't announce it as a mega house. (Even 12 would put it on the long side).
But, there are two primary reasons I don't see them leveraging a lot of Season 3 compared to Season 2 -
One - The demodogs are in Season 2. Demodog puppeters don't have to be lookalikes, so they're easier to cast and can use modified demogorgan costumes.
Two - The kids only engage with the "Mindflayer" monster twice in Season 3 - at Hopper's Cabin and Starcourt Mall.
There's enough in season 2 to justify the majority of a house because that's what Hollywood is doing. Season two returns to Hawkins Lab (which officially had two scenes last year), has the junkyard, a return to the Upside Down, the pumpkin patch, the "exorcism" scene in Hopper's cabin - that's six scenes without even leveraging the fact that a lot more happens at the labs in Season 2 than Season 1.
Season 3, realistically, doesn't have as much. Apart from Hopper's Cabin, there are three scenes where there can be Billy scares (Dinner, The Pool, Starcourt), but that requires casting at least FOUR Billy lookalikes... so I'd only expect one of those scenes... if any. They could leverage the Russians (which could go through the funhouse), but then you can't have Hopper available for any Season 2 scenes. And the Russians are far less visually interesting than anything related to the monsters.
Basically, it's not just about the potential scenes. It's about the logistics of the scenes. Season 2 is a logistically easier season to cast. So, it makes sense that season 2 will take up the bulk of the house.