When my H1B will be effective in case of any transfers?

I am on L1B with company A. I’ve processed my H1B first time with company B and it’s effective from Oct 2012. If company C transfers my H1B now, can I start working for company C before Oct 2012? Please clarify.

Since your H1B is a new one, you can start working on that from 1st October 2012 only.