You can transfer from A to C - before joining B
Hi .. You can transfer from A to C, until you have not joined B.
If you have already joined B, you need to wait for 2 pay slips.
Hope I have answered you.
I am in a similar situation :- How are you going to answer company B that you will not join them.. since you have accpted thier offer and they have spent money for your visa. Cant they take you legally on that.
Also, Can you join a company only before you get receipt number, byt after your firm files the H1 transfer petition. I mean only with the postal verification that the petition is filed with USCIS
Sandy