Learn Italian: Tre amici di Anna sono americani. - Three of Anna's friends are American.

No. In Italian, numbers from two upwards are invariable, meaning they never change their endings. Whether you are counting masculine nouns like amici or feminine nouns, tre stays exactly the same. Only the number one changes to match gender (like uno or una).

Why does the sentence use di Anna instead of an apostrophe 's' for Anna's?

Italian doesn't have an apostrophe-s structure for possession like English does. Instead, you always express ownership using the preposition di (of). So, 'Anna's friends' is always phrased literally as 'friends of Anna' (amici di Anna).

Why isn't the word for American (americani) capitalized?

In Italian, nationalities and languages are not capitalized when they are used as describing words or nouns for people. You generally only capitalize the name of the country itself, like l'America.

Why does americani end in an 'i'?

In Italian, describing words (adjectives) must match the noun they describe. Because amici is a masculine plural noun, the adjective for American must also take the masculine plural ending -i, becoming americani.