H1b transfer- two new employers filing at same time

[RIN409680]

I am working for employer A and have an offer from employer B and employer C. Employer B has filed for H1B and it is approved now. Employer C is in process of H1B filing based on employer A documents and paystub.
Please let me know if I start working with employer B. In 2-3 weeks if I get H1B approval for employer C, can I start working with employer C, after leaving job with employer B.
Also if starting employment with employer B while H1B petition in process for employer C will have any impact on the process and can be denied for employer C.

Please advice.