H1B transfer Queries -->OPT to H1B


I am working for company A in F1 OPT and company B filed my H1B and it got approved recently.My status got changed from F1 OPT to H1B.I have few queries, please provide some information on this.

1)Do i need to quit company A and join B immediately or is there any time limit ?

2)If company A applies for my H1B transfer and got it denied.What happens to my H1B from company B,if they havent withdrawn it.

3)Can i apply for H1B transfer from company A without working for company B?