Rashi on Numbers 10:33/רש"י על במדבר י׳:ל״ג

1דרך שלשת ימים [AND THEY DEPARTED FROM THE MOUNT OF THE LORD] THREE DAYS’ JOURNEY — A distance of three days’ journey they miraculously travelled in one day, because the Holy One, blessed be He, desired to bring them straight away into the Land (cf. Sifrei Bamidbar 82).

2וארון ברית ה' נסע לפניהם דרך שלשת ימים AND THE ARK OF THE COVENANT OF THE LORD WENT BEFORE THEM IN THE THREE DAYS' JOURNEY — This was the Ark that went with them whenever they waged war and in which the broken Tablets were placed. It traveled in front of them a distance of three days’ journey to prepare for them a proper place for encampment (Sifrei Bamidbar 82; Talmud Yerushalmi Shekalim 6:1).