I get you're trying to be consistent, and I'm glad we can see the end of this implied rule stuff. It's good we're transitioning to a fully written down rulebook.
However, the 3 second rule creates inconsistency. No one is following this rule. If you ran at a CT for 3 seconds after the warden is killed, you'd knife them every time. If you ran at CT in the time it takes for there to be a new warden, plus the orders to be given, plus 3 seconds, the round would be over every time. If you ran at any CT during a freeday and waited for 3 seconds after his command to stop, he'd already be dead. These situations don't occur because people don't follow this implied rule everytime, and if we do now enforce it, there will be inconstancy and imbalance.
Players and admins already use their judgement of what is reasonable. As I said, there is no way to judge 3 seconds in-game, no admin is looking at the clock to count seconds. Admins would be expected to make a judgement here, which would be way more inconsistent than the rule I propose.
Further, when someone is typing, there is no rule on how long they have, so admins are making a judgment on this too.
Judgments are more consistent because they take into account context and don't rely on arbitrary times and rules with loopholes. It's used for legal courts, and indeed, many eGO rules, because it's an effective and consistent way to administrate.