The best way to avoid this problem is to start the company yourself. Put everything in your name regardless of your skill set. Buy the domain, incorporate the company, get a basic site up, get a logo, create the twitter account, etc. Now you have complete control over your company and the percent ownership you retain: 100%. This gives you the leverage to make someone a co-founder based on performance, not initial enthusiasm and an idea.

If you already formed your entity then things are going to get a little dirty. First, talk with the other co-founder that is pulling his weight and write down everything that you guys do. Write down everything that you two are responsible for that the third guy can never catch up to: professional relationships built, developer stuff, etc. Write down everything the slacker is responsible for and what little he's actually done. Next, talk with your supporting co-founder about how to split up the company. Should the slacker get 10%? Less? What does he bring to the table? 15+ years of experience isn't worth 1% if it doesn't bring any tangible value.

Then the dirty part: confront Bob. Read Dale Carnegie's "How to Win Friends and Influence People" first. The more aggressive you are during this conversation the uglier the outcome is going to be. Be honest with him, but don't say "You do nothing." Say things like "I feel like I'm getting hosed here. I do 50% of the work for only 33% ownership". Notice the difference in perspective? You're not blaming him for anything. Go over everything you and Fred have done. Ask Bob what results he has to show for his work. Negotiate terms and let him know that work has to get done.

If that doesn't go well, depending on your operating agreement, boot him from the company or dissolve it and start a new one.

Hope that helps and good luck!