Web to Lead from Multiple Pages

On my website

 I have form A on first page.

Then Form B on second Page

Form C on third Page

I am creating Web to Lead from multiple pages.

custom field is form & type is Picklist (hidden - not visible in form )

In salesforce I need to know Lead generated when someone fills form A

or Form B or Form C

Question--- How can I get picklist value A, B or C in salesforce ?