I just add an empty file dummy.capx to the zip. Make sure to mention in the title and/or description, that your project is for Construct 3 only!
Thanks for your response. I was hyped for your tip, but sadly it didn't work... My zip has these files :
but he keeps telling me "You zip archive is missing a required file. Must include at least 1 .capx" when trying to upload the zip...
It create the projet, but missing the "version" (ie, the default version game I guess).