If I want to use a theme for 3 different websites (for clients), do I need to buy the theme 3 times?

I'm not sure I understand your question. Divi isn't really a framework, it's just a very flexible theme. We, the agency, pay an annual subscription fee to build unlimited sites on Divi. Many of those clients have hosting and maintenance agreements through us. As part of that, we'll keep the Divi license paid.

The Elegant Themes membership is a no-brainer for WordPress professionals. You won't find a more value-packed deal anywhere on the net. For just $89 per year, you can build as many websites using our products as you like. That includes commercial projects and client projects too! We have customers that run their businesses on Divi, building hundreds of client sites every year for only a single payment of $89. You won't regret this purchase. — From Elegant Themes

For clients that leave our agency we don't promise any ongoing support or updates. They're welcome to purchase their own license to get updates and support directly from Elegant Themes.

Keep in mind, this is very different than building a custom theme. The build time is much shorter, design and functionality customization is more limited and the client pays much less. We also build custom themes (and plugins) for clients but most of ours don't need it. They just need something "informational" "that looks nice".

As an agency, we would rather make our money from on-going services (like support, SEO, AdWords, email marketing and social media) than a larger up-front build fee. It's better for us and we think it's better for the client too.

/r/Wordpress Thread Parent