Translation:The investor drinks the third wine.
You are on the right track, but for tasting wine(s) the Germans use more appropriately: "verkosten", "probieren", and "schmecken".
The tasting is: "die Verkostung".
Good sentences would be: "Der Investor verkostet den dritten Wein." or if he just drinks: "Der Investor trinkt das dritte Glas Wein."
I think the basic idea is that Der becomes Den whenever the masculine noun is the last object in the sentence, aka the accusative. So essentially whenever there is a sentence that is talking about something happening to a noun, and if the noun is masculine, you use den.
Example: Der Mann isst den Apfel.
This is also why Einen is used instead of Ein. Exactly the same rule applies to masculine nouns.
Der Mann isst einen Apfel
(Disclaimer: I am a beginner in German too and so this might not be the most correct explanation but its worked for me so far)