Ubiq, there are many answers as to why it’s not mentioned in the rif, rosh and rambam’s yad. One of them, which my rebbe rav belsky said, is that they aren’t dinim, but they are realities that chazal warn us about. If we do X, we will he hunted, like the gemara says, as animals in the field. Slaughtered. And that’s precisely what happened.
Chazal teach us how to live as individuals and as communities. The 3 shvuos are part of that mesorah; a very important part. But they aren’t dinim, according to rav belsky, so they aren’t brought in halacha seforim very often. That in no way means we can ignore them – chazal say it, after all.