Learn Icelandic: Þetta eru þrjár krukkur.

In Icelandic, the numbers 1, 2, 3, and 4 change their ending to match the gender of the noun they are counting. The word for jar (krukka) is feminine. Because we are counting feminine objects, we use the feminine form þrjár instead of the masculine form þrír.

Does Þetta mean "these" here? I thought it meant "this".

Yes! When you are introducing or pointing to things, Icelandic uses the fixed word Þetta for everything, no matter the gender or how many things there are. So, Þetta er means "This is", and Þetta eru means "These are".

If Þetta doesn't change to a plural form, why is the verb eru (are) plural?

Even though the pointing word Þetta stays the same, the verb still has to agree with the actual things you are talking about. Since you are pointing out three jars, the verb must be in the plural form (eru), rather than the singular (er).

How can I tell that krukkur is a feminine noun so I know to use þrjár?

You can often guess the gender by looking at the basic singular form. The singular form is krukka (one jar). Nouns that end in -a in their dictionary form are almost always feminine. The plural ending -ur is also a very common pattern for feminine words.