my application works only with python3 and it use QT, can I participate to Ubuntu Application showdown?
Right now I don't know how to create the package... Quickly doesn't work with python3
Yes, you can participate.
Quickly is not necessary to participate in the Ubuntu App Showdown, but make sure that required Python 3.x dependencies are present in the default repository of Ubuntu 12.04.
The rules are here. You will need to upload your app to launchpad and create a PPA.