Besides the other answers, we can also apply the discussion at to top of the daf Sotah 10a (Artscroll 10a1) to this situation. The gemoro asks how could Shimshon attack the Pelishtim against the covenant between Avimelech and Avraham. See note 3 in the art scroll 10a1 which explains that since the Pelishtim violated the oath, it was null and void. Similarly, since the nations have violated their portion of the "three oaths" the entire oath structure is now null and void.
Note that I am giving this reference because it is the current daf hayomi and it seems to apply to other cases of an oath. We also see in maseches Nazir that an oath that is annulled (or violated) in part becomes batul completely.
Note also that the gemara reference to the "three oaths" actually considers them as one unit rather than three separate oaths.
Wikipedia gives a number of reasons which includes
Although the Three Oaths were obligatory in the past, the gentiles
violated their vow by excessively persecuting the Jewish people.
Therefore, the validity of the two other vows has been nullified.
Religious Zionists point to a specific Midrash warning that if gentile
nations violated this oath, then "they cause the End of Days to come
prematurely". This has been interpreted to mean that Israel's
re-establishment would be implemented sooner than originally intended.
With atrocities against Jews throughout history, and especially after
The Holocaust, the Jewish people were absolved of their part of the
Oaths. Those who hold this position often rely on the Shulchan Aruch
which states: "two [persons] who have taken an oath to do a thing, and
one of them violates the oath, the other is exempt [from it] and does
not require permission." As a result, the ban on mass-immigration
to the Land of Israel became void, and Zionism and the State of
Israel arose as a direct result of the breach by gentile nations of
 Shulchan Aruch, Yoreh De'ah 236:6