Learn Finnish: Kolme ja kaksi on viisi.

In Finnish, basic math equations treat the numbers as abstract concepts rather than physical plural objects. Therefore, you use the singular on just like you often say 'three and two is five' in English.

Is ja (and) the standard way to say 'plus' in Finnish?

For simple everyday addition, using ja is very common and natural. If you want to use the strict mathematical term for 'plus', you can just use the word plus! Both are perfectly fine.

Are the numbers kolme, kaksi, and viisi in their basic dictionary form here?

Yes! When you are just stating numbers or doing simple math equations like this, the numbers stay in their basic dictionary form. They only change endings when they are describing the amount of a noun in a different grammatical case.

Does on always mean 'equals' in math?

Using on (is) is the most natural way to express 'equals' in everyday speech. In a strict math context, you might hear on yhtä kuin (is equal to), but for daily life, on does the job perfectly.