Doesn't matter, irrespective of whether or not you are working for them yet, the documents that needs to be presented to USCIS is the same - they file the labor providing an occupational code, get it approved, file for H1B by providing docs to prove why this job description require a foreign national and as to how your educational and work background fits this job and what the individual's job duties are. If you are working for them already, they will establish pay stub otherwise no. Anyways, none of this pertain to whether or not you are already working for them. To avoid waiting for 5+ months to know the result, have the employer file it in Premium. Infact I would suggest accepting offer from both B and C, have them file for H1B in premium (you can have multiple petitions filed at the same time), see which employer can get it approved without RFE and join them. Lets say you get RFE with both, then see who clears it first, join them.