Can I have an H1B transfer from Company B to C while working for A?


I am currently working for an employer A on Student OPT (Optional Practical Training). This year Company B has filed for an H1B and its under process and I plan to start working with them from 1st October 2013. I am currently being interviewed by Company C and I can expect an offer from them soon.
-Can I transfer my H1B(if approved) from company B to C before October (before starting work with company B)?
-Will I need pay stubs from Company B (I will only start my work from 1st October 2013)?Can I show pay stubs of my current employer A and get a transfer from B to C?

I'll appreciate it if someone can answer my questions. Thank you :)
  1. Yes, but only after B’s petition has been approved

  2. You will have to show that you are mainintaining legal status in US. If filed prior to Oct, you need to show OPT status proof (i.e. A’s payslips), and if filed after Oct then you need proof of OPT and H-1 status (i.e. A’s paylsips until Oct and B’s payslips thereafter). At any point, you need 3 most recent payslips which can either be of only A or only B or combo of both.