What concerns do you have about the job?
I don't think it's a good idea to mention any concerns. If it's so serious just don't go for that job otherwise it would be quite light to be avoided. Agreed?
So, you may confidently say "I don't have any concerns at all. I believe I've the right skills and abilities required to be successful in the position. I'm looking forward to the challenge and I believe I'll stay focused to prove my worth... blah blah :-)"
What concerns do you have about the company?
Again, no concerns for the same reasons. You may say "I don't have any concerns about the company. I've researched about the company and found it quite good in terms of work, environment, performance based appraisals & promotions, etc. So, no concerns at all. Just looking forward to have a long association with it"
How soon will you start contributing to the company?
You may say "Well... I don't have enough idea of the complete set of responsibilities, the trainings I need, etc. associated with the position. So, I don't think I'm a good position to comment on it as of now. But, I can assure that my learning curve and time taken to start working on the projects will be as per what the company expects out of an new recruit, if not better."