Learn Finnish: Sanoitko, että pakkasit kolmet uimahousut? - Did you say that you packed three pairs of swimming trunks?

The word for pants, housut (and by extension uimahousut), is a plurale tantum word — a word that only exists in the plural form. When you count items that are inherently plural like matched pairs or multi-part objects, you must use the plural form of the number. Therefore, three pairs of swimming trunks is kolmet uimahousut, not kolme uimahousua.

Normally, numbers force the following noun into the partitive case. Why isn't it kolmet uimahousuja?

Singular numbers do force the following noun into the partitive singular (like kolme autoa). However, plural numbers like kolmet act differently: they simply agree with the noun's case. Since the swimming trunks are in the nominative plural, the number matches it as kolmet.

Since kolmet uimahousut is the direct object of the verb pakkasit, shouldn't it take an object case marker like the genitive 'n'?

It actually is in the object case! For plural nouns, the accusative case (the total object case) is identical in form to the nominative plural. There is no separate '-n' marker for plural objects, so because you packed the total amount of trunks, the plural object looks exactly like the basic plural form.

Why does the verb pakata (to pack) become pakkasit with a double K?