We think about whether we should convert a quite large python web application to Python 3 in the near future.
All experiences, possible challenges or guidelines are highly appreciated.
For each third-party library that you use, make sure it has Python 3 support. A lot of the major Python libraries are migrated to 3 now. Check the docs and mailing lists for the libraries.
When all the libraries you depend on are supported, I suggest you go for it.
Make the Move to Python 3 - Best practices