I had applied for my H1B transfer from company A to company B. And I got query asking for last three months paystubs and Client letter. I had provided the paystubs but I didn't get the client letter. Though I got letter from primary vendor. And finally I got a denial notice. That denial is against the company B. It has three detailed reasons explaining the reason of denial.
My question is should I go back to Company A(they have not cancelled my H1B) or should I look for another employer and file for new transfer.?
It appears you had already started working for B. The denial puts you out of status. In order to go back to A, it is my view, you will need to go back to your home country, get a new H-1 visa stamp and come back (assuming A has not revoked your H-1). If they have revoked your H-1, they will need to also reapply for your H-1. Also search for "unlawful presence" on my blog or on immigration.com.