ionic 3 publish app as PWA (Progressive Web App)

Don't bother with Cordova for a PWA. Just use npm run build --prod and upload the /www folder.

PWA is more of a collection of concepts, so it's not just "on" or "off". At a minimum though you probably want to add a manifest file so you can make it more "app like" by hiding the browser frame, setting your icon, app name etc. Uncommenting the line to add service worker doesn't magically make it a PWA if you haven't put anything "useful" into service worker (which you may or may not need to do, depending on how your app works). Also note that you will need HTTPS to use a service worker.

You will also need to manually remove the line from index.html that includes cordova.js (which will be a 404 error if you just upload /www).

Run

ionic cordova platform add browser

ionic build browser --prod --release

Then go to [project_folder]/platforms/browser/www and copy the content at you http server.