Ionic 3 Project does not have config.xml
From ionic cli v4, you need to set cordova to your ionic app if you are creating an ionic cordova application. config.xml is used by cordova platform.
You can enable the integration after creating the app as well according to documentation.
ionic integrations enable cordova
If you cannot recover or don't have a config.xml file here is a solution I found from,just run ionic integrations enable cordova --add and if it asks to overwrite config.xml, give yes and if it asks to overwrite resources, give no because it will overwrite your resources folder and you will lose many important things
create new project and copy config.xml file and create a new file with name config.xml and paste in it