if you upgrade an existing site to 3.0 then yes, it will maintain backwards compatibility.
If you install 3.0 on a new site and then import a template you may see some changes. The main ones are:
1. Box-sizing: border-box is now applied to the Theme. So in the past a 1100px container with 40px padding allround would be 1180px wide. In 3.0 the padding is kept within the container and the size will now be 1100px.
2. Flexbox replaced unsemantic grid. In the old theme you will see CSS classes in the HTML such as grid-50 – so if you used any of the old grid in your custom HTML then you will need to select the Floats option in Customizer > General to keep that function.
Remember we’re still in Alpha – so lots of testing is still be performed 🙂