Sforno on Genesis 6:16/ספורנו על בראשית ו׳:ט״ז

1בצדה תשים, on the side which is wide, for that is the side called צד. The long side is referred to as צלע.

2תחתיים, the lower deck in ships; as in any other ship, the customary lower deck.

3שניים ושלשים, they are similar, parallel to the lower deck.