RetopoFlow 3 beta : https://github.com/CGCookie/retopoflow/tree/b280

1. To get the latest (development) version of Retopoflow, go to https://github.com/CGCookie/retopoflow/tree/b280. Mind the end of the URL, you need to get the latest changes from the b280 branch.

2. Click on the green button "Clone or Download", then "Download ZIP"

3. Extract the retopoflow-b280.zip file and rename the "retopoflow-b280" folder to "retopoflow".

4. You may notice that the "Addon_common" folder is empty.
Go to https://github.com/CGCookie/addon_common/tree/b280 and download the content of the repository (download the ZIP file).

5. Unzip "addon_common-b280.zip" and extract its content to the "retopoflow\Addon_common\" folder.

6. re-zip the "retopoflow" folder and install the addon normally from the Preferences > Addons > Install button.