Learn German: Wir sollten die Fusion dreier Konzerne genau prüfen. - We should examine the merger of three corporations closely.

It is in the genitive case, meaning 'of three'. Normally, numbers do not change their endings. However, when zwei and drei are used in the genitive plural without a preceding article or adjective to show the grammatical case, they take the -er ending to make the genitive relationship clear.

Would it be wrong to say die Fusion von drei Konzernen instead?

No, that would also be perfectly correct. Replacing the genitive with von plus the dative case is very common, especially in spoken German. However, the true genitive form dreier Konzerne sounds more professional and elegant, which perfectly fits the formal business context of reviewing a merger.

Why doesn't the noun Konzerne get an extra genitive ending like -s?

The -s or -es ending is only added to masculine and neuter nouns in the genitive singular (for example, die Fusion des Konzerns). In the plural, nouns do not get an extra genitive ending. They stay exactly the same as their standard nominative plural form, which is die Konzerne.

Is sollten in the past tense here?

While sollten happens to share the exact same form as the simple past of sollen, here it is functioning as the subjunctive II (Konjunktiv II). It does not mean 'we were supposed to' in the past, but rather 'we should', expressing a strong recommendation or polite advice for the present or future.