Need suggestion on when to join another employer on consular processing H1b?

Hello All,

I am an F1 student on STEM OPT which is valid until January 2020.Employer A has filed for H1b and it got approved with consular processing.I have received another offer from Employer B.The suggestion from employer B is to convert as STEM opt and they will file for COS and do the transfer.How far is this good with the current trend.Is 3 pay stubs necessary from the employer A who filed for H1B in order to apply for H1B transfer?