What should I do if I only worked for 3 months and did not get a w-2 form

Contact the employer who did not provide you with the W-2 and request that it be sent to you. Or you can go pick it up so you can complete and file your return by July 15th. 

your "employer" may have treated you as an independent contractor. (IC)    if medicare and social security were deducted from your payments you were being treated as an employee and should have gotten a w-2.  on the other hand, if they weren't being deducted, you were an IC and now need to file schedule C. if you were paid over $600 you should have gotten a 1099-misc rather than a w-2.   if you no longer have your paystubs, you can see if the IRS has a transcript of your w-2 or 1099-misc (if that was filed).   

 https://www.irs.gov/individuals/get-transcript

WAGE AND INCOME TRANSCRIPT