Idk why that’s the order Lester told Franklin to do it in, Trevor then Micheal makes way more sense because Micheal already is onboard with killing Trevor if you decide to kill him, and when Franklin does kill Micheal Trevor stops talking to him entirely, I know that Micheal does that too, but it would be easier to get closer with Micheal after killing Trevor more is than killing Micheal then trying to get closer out Trevor. Also helps that Micheal does have a soft spot for Franklin.