I think there is a rule somewhere about advantage not being able to stack, and the wording of the feat is meant to sidestep questions about that. So with that interpretation id say go for it.
You're exactly on point. You would reroll the lowest of the original two and then use the highest. So regardless of whether you roll two and then reroll the lowest or just roll 3 - the result is exactly the same. You've rolled 3 dice and taken the best either way.
Without other re-rolls does it matter if I roll all three dice at the same time, to not slow down combat?
I'm always re-rolling the lower of the original 2 dice.
The only 2 reasons I can think of won't change anything as I'm not Lucky or a Halfling.
I think that is fine. If your DM doesn't have a problem with it, go for it.
I think there is a rule somewhere about advantage not being able to stack, and the wording of the feat is meant to sidestep questions about that. So with that interpretation id say go for it.
You're exactly on point. You would reroll the lowest of the original two and then use the highest. So regardless of whether you roll two and then reroll the lowest or just roll 3 - the result is exactly the same. You've rolled 3 dice and taken the best either way.
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