Learn Russian: Мы позвали троих ювелиров.

Троих is the animate accusative form of the collective numeral трое (a group of three). With masculine nouns referring to people, like ювелир (jeweler), Russian allows you to use either the standard cardinal numeral (трёх) or the collective numeral (троих). Both are grammatically correct here, though the collective numeral subtly emphasizes that they were summoned as a single group or unit.

What case is троих ювелиров in?

It is in the accusative case, because they are the direct object of the verb позвали (we called whom?). However, because jewelers are animate (living people), the accusative plural takes the exact same forms as the genitive plural. That is why we see the genitive-looking endings -их and -ов.

How exactly do you decline a collective numeral like трое in the oblique cases?

In oblique (non-nominative) cases, collective numerals decline just like plural adjectives. Here, троих uses the standard plural animate accusative/genitive adjective ending -их, just like you would see in хороших or больших.

Wait, does позвали mean we called them on the phone?