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."
According to the Olde English Translator, gewunen doesn't mean defeated or destroyed. It's the present subjunctive plural form of the verb gewunian, which means "to remain continue stand to habituate oneself to" which puts me in mind of "tolerate" or "get used to."
Well, the Killing Curse works on animals, or as Professor Quirrel puts it, "anything with a brain," so that's gotta count as some kind of evidence that AK works on infants. They should possess the same "innocent outlook" an infant has.
Plus, I thought it was part of canon that Death Eaters were known to have Avada Kedavra-ed whole families during the first war on Voldemort. We don't know explicitly of any other attempts to Avada Kedavra infants, but it stretches the bounds of plausibility to think that nobody else has ever tried to Avada Kedavra a baby in the history of the curse. Distraught mothers trying to kill their babies is common enough (too common), and AK would probably seem like an attractive option to such witch mothers. No pain, no struggle, just death. That's not to mention the infanticide that happens during wars and feuds.