H1B Multiple petitions for change of employer

I am working with employer A. Employer B & C have filed separate petition for change of employer for me. After receipt notice, I join employer B. But I find that employer C is better for me. After receipt notice, I now join employer C. Do I have to file fresh petition for change of employer from B to C or earlier petition for change of employer from A to C will be valid.

How long were you at Employer B? Did they run payroll?

This is a hypothetical question which will most likely happen with me next month. Actually I have to join C who is willing to give me joining date after 3 months.To avoid going out of status, I shall have to join B. B will run the pay roll. What happens if I remain with B for 1 month . What happens if I remain with B for 2 months or maximum 3 months.