Learn Finnish: Kolme plus kaksi on viisi.

In Finnish mathematical equations, the verb is always in the third-person singular (on). The calculation itself is treated as a single abstract concept rather than a plural group of physical objects.

Why are the numbers here in the basic nominative form rather than the partitive case?

When you are stating a math problem and just naming the numbers as concepts, they stay in the nominative case. You only use the partitive case when counting actual nouns, like saying three apples (kolme omenaa).

Could I use the word ja instead of plus here?

Yes, in casual speech it is very common to use ja (and) for simple addition, such as kolme ja kaksi on viisi. However, plus is the standard mathematical term.

Are there other ways to express 'is' or 'equals' besides on?

While on is the most common choice for everyday math, you might also hear tekee (makes) or the more formal mathematical phrase on yhtä kuin (is equal to).