1. Grid System: Bootstrap uses a 12-column grid system, while Pure uses a responsive grid that is not based on a fixed number of columns. Semantic UI offers a 16-column grid system with a better control over element placement compared to Bootstrap and Pure.
2. Customization: Bootstrap provides a lot of pre-designed components and styles, making it easy to create a website quickly. Pure offers a more minimalistic approach, providing only essential styles, requiring more customization for unique designs. Semantic UI has a theming engine that allows easy customization of the design system.
3. JavaScript Components: Bootstrap includes JavaScript components like modals, carousels, and tooltips, offering a wide range of functionalities out of the box. Pure focuses solely on styling and layout, lacking pre-built JavaScript components. Semantic UI provides a rich library of ready-to-use JavaScript behaviors and components for a more interactive user experience.
4. Community Support: Bootstrap has a large community and numerous resources available, making it easy to find solutions to problems and access third-party plugins. Pure has a smaller community but is maintained by the team at Yahoo, ensuring ongoing support and updates. Semantic UI, although growing, may have fewer resources and community support compared to Bootstrap and Pure.
5. File Size: Bootstrap has a larger file size due to its extensive features and components, which can affect page load times. Pure is lightweight, focusing on minimalism and performance optimization, resulting in smaller file sizes. Semantic UI strikes a balance between the two, offering a rich set of features without compromising on performance.
6. Documentation & Learning Curve: Bootstrap has comprehensive documentation and a large number of tutorials available, making it easy for beginners to learn and use. Pure has simpler documentation but may require more effort to set up custom styles. Semantic UI has detailed documentation with interactive examples, but its unique syntax and components may have a steeper learning curve.

In Summary, Bootstrap provides a feature-rich framework with extensive community support, Pure offers a minimalistic approach with lightweight performance, while Semantic UI strikes a balance between the two with rich features and customization options.