Then, once we choose Option B there down will be shown few more input-s (like i said above: B1,B2,B3) ...
Yes, I know. And I wanted you to show us how you will present B1,B2,B3 and what kind of fields they are, etc., etc.
Don't make us create a dummy HTML page for you just to demonstrate this simple code, only to find out that our dummy HTML isn't anything like what you wanted.
Give us the HTML with *ALL* fields (A1,A2,A3 as well as B1,B2,B3) *FIRST* and then it's easier to show you how simple the code is.