On which page of the manual are those diodes?
Anyway, I don't know how three diodes suddenly failed. As I understood, they failed open, which even more suspicious. Most of the time overloaded diodes fail short (unless bound wired melted
https://electronics.stackexchange.com/questions/77712/can-a-diode-fail-to-open-position). But if it's a diode in keyboard matrix, it's not supposed to carry any significant current. What's even more suspicious is that it seems only diodes failed. I'd say diodes more robust than a typical IC.
I'd replace diodes and checked the power rails (and may be installed a current limiting resistor, but that's a hack). May be there is a problem with one of the rails so somehow voltage on one of the rails got lower/higher than needed and there was significant current through these diodes. But if it's a keyboard diode then diodes can only conduct when a key is present...
May be there was an ESD zap, and diodes are/were too sensitive, but so far I like power rails theory more as power rails have more energy to destroy diodes open.