Breakdown of Мы с троими ювелирами смотрели на эти грани.
мы
we
этот
this [f: эта, n: это]
на
on, at / by (transport) / to (a place) / to (in answer to)
смотреть
to watch / to look at / make sure (in a warning)
Questions & Answers about Мы с троими ювелирами смотрели на эти грани.
Why use the collective numeral троими here instead of the standard numeral тремя?
Since ювелир refers to people, you can use the collective numeral трое. The preposition с requires the instrumental case, and the instrumental form of трое is троими, which declines just like a plural adjective. Using the regular cardinal numeral тремя would also be perfectly correct here.
In oblique cases like the instrumental, does the collective numeral still force the noun into the genitive case?
No, that rule only applies in the nominative or inanimate accusative! In all other oblique cases, the numeral and the noun simply agree in case. Because the preposition с dictates the instrumental case, both the numeral (троими) and the noun (ювелирами) take the instrumental plural endings.
Why does the sentence use Мы с to mean Three jewelers and I?
Russian frequently uses this inclusive pronoun structure. The phrase мы с plus the instrumental case literally means we with someone, but it is the most natural way to express someone and I. The verb смотрели takes the plural form because the core grammatical subject is мы.
Why is эти грани in the accusative case here?