Here are the 3.x versions of the Develop labs.
- Build a Starter Map
- Add Feature Layer
- Add Vector Tile Layer
- Style Feature Layer
- Style Feature Layer with JSON
- Style a layer popup
- Add GeoJSON Layer
- Search with a Widget
- Search with a Query Task
- Client-side Buffering
- Client-side Buffering with UI
- Smart Mapping
- Search with HTML5 Location
- Create a JS API 3.x WebMap App
You can see a high-level overview of the differences between the 3.x and 4.x JS APIs here and a more detailed comparison here.