John Calvin Commentary Numbers 7:3

"and they brought their oblation before Jehovah, six covered wagons, and twelve oxen; a wagon for every two of the princes, and for each one an ox: and they presented them before the tabernacle." — Numbers 7:3 (ASV)

And they brought their offering before the Lord, six covered wagons. These wagons were dedicated for the transport of the tabernacle, for its pillars and many other parts of it could not be carried on men’s shoulders. Therefore, they are said to have been covered, so that the things placed in them would not be exposed to the rain. For it is by no means suitable to suppose that they were litters;⁴⁰⁰ and, in fact, a pair of oxen was assigned to every wagon. It is quite clear, then, that the materials of the tabernacle were placed in them when they were traveling from one place to another.

This offering is stated to have been made before the Lord, and then before the tabernacle, but the meaning is precisely the same, for God had, as it were, put on that face in which he might be seen by believers.

Regarding what follows, But the Lord had spoken to Moses, etc., I interpret it in this way: that God had required this tribute from the people. I have therefore considered it right to translate it in the pluperfect tense, whereas others translate it, The Lord said to Moses, as if Moses had not been ordered to receive it before it was actually presented by the princes and the people.

Indeed, it is probable that the number of the wagons was not accidentally determined, but by a precise calculation of the things they were to carry.

⁴⁰⁰ So Vatablus, quoted in Poole. “In Leviticus 11:29, (says Ainsworth,) צב (tsab) is a tortoise, so called from the shell that covereth it: accordingly here they may be called עגלת צב, (gnegeloth tsab,) waggons of the tortoise, (or of covering,) because they were like to a tortoise, covered above.”