"Three serious economists organized this meeting."
Translation:Три серьёзных экономиста организовали это собрание.
Why is it "серьёзных экономиста" instead of серьёзного экономиста or серьёзных экономистов?
Oh, okay. From Wiktionary:
"While the noun in a nominative construction is in the genitive singular following два or две, an intervening adjective or adjectival participle will be in the genitive plural or nominative plural."
That applies to три and четыре as well. And one last bit of explanation from Wiktionary:
"... in Old Russian, два governed the dual number, and the old nominative dual form was the same as the masculine genitive singular. When the dual number was lost in Russian, the form came to be considered the genitive singular that it resembled. This was extended to include the numbers три and четыре, and eventually it was applied by analogy to feminine nouns as well."