I'm just wondering because I've found multiple third evolutions in Pokemon Black.
It's possible to find wild Azumarill (Victory Road), Beautifly, and Dustox (routes 205, 224, 229, and Eterna Forest). It's also possible to find wild Gengar in Old Chateau if you have a gen 3 game on your DS. source
No, you cannot. Pokemon BW are the first games to allow catching wild third evolution pokemon. Hope this helps!
