Um... no pay for working with 2 guys with an idea... full 33%. You have talent, they have nothing but an idea (everyone has ideas, they're mostly worthless), you should be treated as an equal.
Wrong way to look at it. They aren't looking for developers... that requires paying a salary. By offering no salary they're looking for another founder, which means full share. If they want more founders, they all contribute shares to bring then next on board as well.
How the hell does everyone equate no-salary == founder?
An employee (that's what this guy can be) can do work for whatever compensation model both parties mutually agree on.
The founders of this company are offering him employment terms. It's difficult to say if those terms are truly fair or not because none of us have enough info to judge fully.
I've known people who have worked for stock, "expenses" (leased apartment, etc), product trade (I'll write your docs if you'll host my server farm), and all sorts of other arrangements.
I think that too many people here are getting hung up on trying to achieve "founder" titles.
In this case, they are looking for a developer, not another founder.
I agree that often no salary != founder. For instance, a recruiter, legal advice, or short term contractor might agree to a small equity share instead of a payment. Without more information, we can't be sure about the current situation.
But it sounds like there are only 2 other founders, no employees, and no product. I'm not sure if there is any funding or investment at this point either. Because the original poster would be the first technical employee, I'm going to guess there hasn't been much prototyping either.
Considering that this guy would be the third in the company and would be the first guy with a technical background, I think it sounds like a no salary == founder situation.