The word "transfer" is a bit of a misnomer - your work authorization remains valid until it's revoked, so you can go back to employer "A" without "transferring" your H1B back to them. You can "transfer" to company C as long as you meet the requirements for transfer and they are an eligible employer, even if you have never worked a day for Company B. If you want to speed things up, pay for premium processing when you apply for the transfer.