Here's the closest I could come up with, but it is a two-button workflow. Essentially a MarkIn -3 button and then a MarkOut button.

Write a script: First function to ReplayMarkInOutLive for 3 seconds (this will mark to the -3 seconds from when you press the button). For the second function ReplaySelectLastEvent. Assign this script to your MarkIn -3 shortcut button.

On the second shortcut button assign ReplayUpdateSelectedOutPoint. That will update the outpoint of the selected event when you press the shortcut.

There may be a way to put this in one script that will wait for a second button press to run the ReplayUpdateSelectedOutPoint function. However, my scripting skill level is not that high yet.

Best of luck,
C