In order to answer your question there are a couple of important data points that need answering: 1) Are any of the three co-founders engineers? Or are you exp

In order to answer your question there are a couple of important data points that need answering: 1) Are any of the three co-founders engineers? Or are you expected to run engineering? 2) How senior are you? Will you be expected to take on a leadership role or, after they hire a couple more folks, will you just be one of the team? 3) What is the fundraising situation for the company? 4) Has the company released a product to the market? If so, how much traction is there? The first two questions go towards answering how important you might be to the organization. Obviously more important -> more equity. The last two questions go towards evaluating how much success has been achieved already. More success -> less equity. Though less isn't necessarily a bad thing here as you'll be joining a company with a greater chance of success.