Can I use a Link-2 Dark Dragon monster and another Dark Dragon monster to summon Vorticular Drumgon or do I need the 3 Dark Dragon monsters?
No, you can't.
At first one may think you could. Extracting from the rulebook, pages 14-15, emphasis mine:
Normally, 1 monster equals 1 Link Material. But when you use a Link Monster as material for another Link Summon, you can count it as either 1 monster, or as a number of monsters equal to its Link Rating.
So, one can indeed use a Link-X monster and count it as X monsters.
However... Vorticular Drumgon has a specific requirement for their materials to be "3 Dark Dragon Monsters". Therefore, you are limited to use exactly 3 monsters. The ruling also covers that part and indicates so in the same pages (emphasis mine):
Remember that when you Link Summon, you need to meet 3 goals:
The amount of materials you use matches the Link Rating (Link Monsters count as 1 OR their own Link Rating).
The number of materials you use meets the number specified in the materials line or “recipe”.
The materials match any other requirements in the materials line (such as only Effect Monsters, or only a specific Type of monster).
So, technically you can use a Link-2 Dark Dragon for the summon, but you would need other 2 monsters that meet the same requirements (being Dark Dragons), as Drumgon explicitly asks for 3 monsters (no more, no less).