In IONIC 3 LoadingController you can set the content or text in starting. But if the loading is for getting information from many place and you want to display how many percent was completed or the process until which step.
this.loading = this.loadingCtrl.create({content: 'Please wait...'});
The above example is showing the loading controller inside the app. and now if you want to change the content you can refer below example
this.loading.setContent("Updating user");