Good catch, I wasn't even thinking if there were a different, related verb that might be used there, nor of the particular grammar. That's just where that form gewunen showed up in the translator.

If the verb is winnan or gewinnan, the past participle would be gewunnen. In either case, the sense is conquering to obtain, or alternatively resisting, struggling against, enduring or suffering. And there are less ambiguous words to use if the sense was that Death would be defeated and eliminated, i.e. destroyed, or even mastered or overcome.

In other words, it still looks ambiguous enough to me that it could mean that "...three shall be their devices by which Death shall be tolerated."