Multiple H1B Transfers

[SaxaphonePlyr]

Hi,

I work at Company A. I was thinking about changing jobs and started attending interviews at company B and company C. I got an offer from company B, and I accepted the offer from company B, since company C was taking longer time to arrange interview.  The H1B visa transfer process with company B was started and it got approved as well. After H1 transfer got approved from company B, I  finally got offer from company C as well.  Now,  the offer from company C is much better in terms of many aspects, and I want to accept it. I have the following questions:

• Should I inform Company C about the H1 transfer that got approved with Company B ?
• When is it the best time to tell Company B that I want to walk away from their offer. Can I tell them before the H1 transfer with Company C is approved ? Honestly I want to tell them as soon as possible with the good intent of not holding them for longer. But I am not sure if telling Company B early before the Company C's H1 transfer is approved might hurt the H1 transfer.

Please help!