I, too, think three brewing stations in Vallaki's vicinity are more than enough, not counting the underground ones. The issue is rather people not knowing they exist, aside the most visited one next to the fishing lodge. The vast majority of those I've personally informed about the other two had no idea they're there. Spread the word IC!
Speaking of IC, I'd argue that several people gathering to brew in one cauldron is more of an RP opportunity than if there were multiple ones. Those waiting need to keep busy somehow, after all. Yes, I do agree that it can be time consuming and frustrating at times, I've been there myself more than once. Even so, spamming even more cauldrons in the area (which will probably end up remaining unknown to most people anyway) isn't necessarily the solution either.