Learn Finnish: Tuo kilohinta on kolminkertainen. - That price per kilo is triple.

The suffix -kertainen means '-fold' (as in threefold). To attach it, Finnish uses a special stem of the number. For kolme, this stem is kolmin-, giving us kolminkertainen. This same pattern applies to other small numbers, like kaksi becoming kaksinkertainen (double).

I sometimes see kolmenkertainen with an 'e'. Is that also correct?

Yes, it is! You can form these multiplicative adjectives using either the instructive stem (kolmin-) or the genitive stem (kolmen-). Both kolminkertainen and kolmenkertainen are perfectly correct and mean the exact same thing. However, the -in- versions are generally much more common for the numbers two, three, and four.

Why is kolminkertainen in the nominative case here instead of the partitive?

In Finnish 'A is B' sentences, the case of the describing adjective depends on the subject. Because kilohinta (price per kilo) is a singular, countable concept, the adjective describing it must be in the nominative case. If the subject were an uncountable mass noun, the adjective would take the partitive case instead.

Is kilohinta a standard compound word, and can I make others like it?