Learn Russian: Охранник сказал, что эти трое незнакомцев должны уйти. - The guard said that these three strangers must leave.

While you could grammatically say три незнакомца, Russian strongly prefers collective numerals like трое when talking about small groups of men or mixed-gender groups of people. It gives a natural sense of 'a group of three' rather than just strictly counting individuals.

Why is the noun незнакомцев in the Genitive plural?

Collective numerals like двое, трое, and четверо in the Nominative or Accusative case require the noun that follows them to be in the Genitive plural case. The Nominative singular here is незнакомец, and its Genitive plural is незнакомцев.

If незнакомцев is in the Genitive case, why is эти in the Nominative plural?

The phrase эти трое acts together as the subject of the second half of the sentence, so both words are in the Nominative case. The collective numeral трое forces the noun after it into the Genitive case, but it does not affect the modifier coming before it.

Why is должны plural here?

The word должны is a short adjective meaning 'must' or 'obliged to'. When collective numerals refer to people as the subject of a sentence, especially when paired with a plural modifier like эти, the verb or short adjective that follows takes the plural form to match the group.