1. When I check in in Manchester is this the only time I need to check in or do I have to do this at every airport for the three flights?
You can probably check-in online for everything and print out boarding passes.
2. I assume my bags will follow me and I will not have to pick them up?
Unfortunately it is highly likely that you will have to pick up your bags. This is because it is unlikely that Germanwings and Lufthansa have an inter-carrier agreement.
However if Manchester -> Dusseldorf -> Frankfurt is on Germanwings then at least your bags should be able to magically transfer for the first 2/3rds of your trip !
3. What do I do at Dusseldorf / Frankfurt? Is there a dedicated transit area where I go?
I've never flown to Dusseldorf. But Frankfurt airport is
GIGANTIC, so you may well find you'll have to change terminals (or at least be ready to walk a very, very long way within the same terminal !).
Edit to add:
- The internet suggests Germanwings is terminal 2, and Lufthansa is terminal 1 at Frankfurt, but because you will likely have to pick up your bags at the point you switch from Germanwings to Lufthansa, you'll just need to follow the normal process for leaving any airport (i.e. show your passport, pick up bags, go through customs....)
- Actually, I may be wrong...this document....
https://files.lufthansaexperts.com/l...1.86153.5c5cae ... appears to say :
Currently, Germanwings has interline agreements with LH, OS, LX, SN as well as UA and AC. Coupons from other airlines’ documents that are affected by a rebooking on to 4U will be amended centrally by LH on an LH document.
I would therefore make it clear to the Germanwings checkin at Manchester what the full extent of your itinerary is and explicitly ask them to check your bags through (since the check-in desk person may just be on "autopilot" and only check through your bags half way).