You can always obtain a duplicate copy of your mark sheet if you are not concealing anything. In my case, I had lost all my certificates. I joined a concern after procuring all the duplicates and made some affidavits.
Claiming Unpaid Salary
Regarding working for 3 months, please advise if you have signed the attendance register or if your handwriting is on any of their official papers. If yes, you can very well claim your salary as no one has the authority to take service free of cost. The company will be caught under Section 420 of the IPC. First, take photocopies of all documents as proof. Then, lodge a complaint with the Labour Commissioner of your locality. If no solution is forthcoming, lodge an FIR under Section 420.
But first, openly discuss the matter with the HRD and the boss of the company.
Regards