Hello Bob and thank you for joining our How to Community.
Bob to adapt from 3” cast iron or 3” copper to PVC you would need to use something called shielded strong back coupling. For more information about this product and installation instructions, click on the attached image.
These fittings are rated for underground repairs however it is still advisable to check with your local building department for code approval.
As far as the toilet flange goes ideally once it’s all said and done it should rest squarely on a surface even with the finished floor. I use toilet flange spacers for this application.
Hope this helps.
George
