Why is the Corleone Compound abandoned by part 3?
Michael, in his longtime attempt to become legitimate, has sold his interests in the Nevada casinos (the reason the family relocated to Nevada to begin with) and returned to NYC to better manage his legal empire. (The movie mentions that he cleared his return to NYC with the Commission first and that he has no major interests in organized crime there anymore [the Corleone territories now being under the control of Joe Zasa]).
Okay, so how come it appears to be a wreck?
Was it attacked and destroyed?
I don’t think there is any “logical reason” for the Tahoe compound to have been in such a wreck. It was merely a plot device to compare and contrast the start of GF III with GF II
To hazard a a guess at the OP, Michael “temporarily” abandoned the Tahoe estate when he moved back to New York. The Tahoe estate had a lot of bad memories for everyone
[ul]
[li]Michael’s son Anthony had bad memories[/li][li]Connie remembers “poor Fredo drowning in the Lake”[/li][li]Michael’s overwhelming guilt from ordering the murder of his brother there[/li][/ul]
Michael didn’t sell the Tahoe estate because wanted to reunite his family [maybe even including Kay] back in Tahoe. But the estate was out-of-sight, out-of-mind-- unless or until, the family was reunited.
Michael didn’t want to sell the estate, didn’t want to go back there alone, and didn’t want anyone else to use it, so it just slowly decayed as the years passed.
The memories were just too painful, even for a monster like Michael Corleone.