Good goalie depth in the organization. You always want to have a few developing. Due to the mental aspects of the game, it's hard to predict which players will develop.
The 3 goalie thing shouldn't be a thing. UPL and Reimer in Buffalo. Reimer plays if UPL gets hurt of pulled, and a few back-to-back road games.
When the Sabres have home back-to-back games, paper transaction someone to Rochester and bring Levi up to start a game, then send him back down.
Non-UPL starts:
Road: Reimer against Columbus (October 17), San Jose (November 22), Islanders (November 30), Washington (December 14), Seattle (January 20), Winnipeg (March 23), Philly (March 29), Florida or Tampa Bay (April 12-13)
Levi starts against New Jersey (October 5)
Home: Levi starts against Islanders (November 1), Toronto (December 20), San Jose (March 4), Boston (April 6)
Reimer gets 8 road starts
Levi gets 5 starts (4 home and 1 road)
That should be the going-in plan, then adjust based on UPL's workload. He'd be at 69 games. Saros played 64 to lead the league last season, and UPL played 54 games. I'd adjust about 9 more games to Reimer (primarily road games) and Levi (home games) to get UPL to about 60 games.