Sam2: First, no one “flippantly”‘ ignores anything. Seforim have been written about the sholosh shevuos and whether they are applicable today (see R”Zvi Hirsch Kalisher, amongst others).
As far as what is or is not halacha- we have a shulchan aruch for contemporary issues and, if you ask, we have a Rambam who deals with all halochos. Nowhere in this vast halachic output are the “sholosh shevuos’ mentioned. Even the gemoro has numerous quotations that contradict the “sholosh shevuos” (you cannot sell land to a non-jew.etc…)
In a lengthy “Igeres Teiman”, the Rambam does mention them- yet, when you look at his codification of halacha in Mishneh Torah and,even more explicitly in Pirush Hamishnoayos on Chelek, he describes what Moshiach may and will do and waging war against our oppressors is paramount. Wouldn’t that be against the sholosh shevuos? I think it is pretty conclusive from the Rambam that the shlosh shevuos were, either unapplicable or only valid for a certain time.