Definition P-Completeness
You should really work this out yourself, but here are a few hints.

Define P-completeness just as you say, by taking the definition of NP-completeness and changing NP to P. Then ask yourself which problems are P-complete. For a problem to be P-complete it must be in P. What about the other condition? Can you come up with a problem in P which you'd conjecture shouldn't be P-complete under your definition?

Once you've answered part 2, you will be able to answer part 3, so don't despair if you have no idea what they want in part 3 right now.