Hi
Is 3 hours enough time to collect luggage and clear immigration in Manchester England to then catch train to London ?
I'd say so, but if you are looking to get an advance purchase discount train fare, keep in mind that there is no guarantee your flight lands on time.
You should be through the airport within an hour. There are no direct rail services from the airport to London. Depending on what time you arrive, you would either head north into Manchester city centre and take a train to London from Piccadilly station. Alternatively, take a train west to Crewe and pick up a London train from there.