In the three times A attacked Sasuke, Minato, and Naruto; there is a very brief pause between the moment A arrives in front of his opponent and the moment where he strikes.
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In that short pause, Naruto had the reaction speed and the bodily speed to dodge A.
In that short pause, Minato had the reaction speed to activate FTG and dodge A. (He also flicked a Kunai up, then dodged the first time he dodged him)
The key is the short pause that A makes when he attacks right before he hits his target cause during the pause he is right in front of the target's face.