3/0 is good for a 200A service, assuming it's copper. Single conductors in conduit are acceptable everywhere, as far as I know. Requirements for the conduit from meter pan to panel can vary depending on your specific locale. Usually this is good question to ask your local electrical inspector.
When using copper conductors for a 200 Amp residential service, #2/0 is sufficient.
You CAN use #3/0 but unless you are fully loaded, the upsize is a waste of resources.
Installing type THHN/THWN in a conduit is perfectly acceptable. Check your local requirements and restrictions before using PVC however. PVC would not be appropriate in places that have extreme cold, such as Alaska and some northern areas. The PVC can become quite brittle and shatter in such harsh environments.