I would like to know the USCIS rules on the following situation

Employer A - approved i140 for more than a year
-> now moved to Employer B, let assume B did not file i140 or I wanted to change because I did not like the job after 3 months of employment with B
case 1) Employer B) may have started the Green card process
Case 2) Employer B) did not start the process at all

-> now moved to Employer C

1) Is it possible to switch to employer C in this scenario with i140 approval from employer A (please consider case 1 and case 2 with employer B)
2) in case of 2) during Employment with B, can B file unlimited extension or only first extension using Employer A i140 card is possible?

Appreciate help