We need to update a legacy Ionic 3 project into Ionic 6. Ionicframework has a migration guide from 3 to 4, 4 to 5 and 5 to 6. But the most challenging is migrate 3 to 4.
Migration from 3 to 4 ionic start myApp tab
It creates a blank project in Ionic 6.
So first step: how to create a Ionic 4 project using Ionic CLI instead of 6?
Step 2: What files to change to Ionic 4?
Step 3: How to run a blank Ionic 5 using Ionic Cli?
Step 4: What files to change to update from 4 to 5?
Step 5: What files to change to update from 5 to 6?
Suggestion: To avoid a lot of errors and adaptations and many hours developing, I think you should go directly to the Ionic 6. Start a new App and transfer the logics and page flows to the new App. Instead It can be a headache, If you go from version 3 to 6. There are also many changes on the libraries and plugins installed on the old App compared to a new one.
Update: You can check the Ionic Breaking Changes.