Your proposals do not make complete sense.
Do you have a 3rd company offer, or are you just exploring your options?
Wait until you start Company B before you 'play around' with Company C.
FORMER IMMIGRATION LAW PROFESSOR -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.
If Company B was the company that applied for the H1B transfer, you cannot begin working with Company C without informing USCIS of this change. Company C should file a H1b transfer application for you.
This information is for informational and educational use only and does not establish an attorney-client relationship.