[shamanNS]

For PW3 with a jailbreak you have an option to install Rescue Pack and Coward's Rescue Pack (you need BOTH, CRP is an "addon" on top of RP, so first install RP then CRP) and then you can boot into diagnostics mode without needing serial connection: you just connect the Kindle to charge and select restart option in Kindle UI and it will boot into diagnostics.

That is the "Coward's Rescue Pack" functionality: check if device is charging on restart / reboot if it is boot into diagnostics instead of booting into main OS, if not charging boot into main OS.
It is a nice safety blanket I wish I had on my PW5 that I've managed to brick. Unlike PW3 PW5 doesn't have diagnostics (RP & CRP aren't compatible with newer Kindle models) and also doesn't have serial (at least not on a production board OR not in the same place like on the photos from FCC) points.