Do I have a right to work in the US if I have student visa?
need: Transcripts and recommendation letters from previous school, TOEFL, GRE/GMAT if applying for grad school. If you do not have money to pay for it, have to find whatever is necessary for Financial Aid - proof of no money (bank statement, parent's salary), and have good idea for  "statement of purpose" (wanna make the world a better place, in this specific&realistic way, which requires education)\n\nCan only work for your university, part-time (up to 20hrs/week), and likely in related fields. Maybe can do full-time summer internships and 1-year Optional Practical Training after graduation.
The instructions for the form do allow for some leeway in the "how long do you plan to stay" question.  Still, you should call the office at your school that produced your I-20 form to make sure that they didn't make an error.  As you read, the I-20 specifically asks about the "normal" term of study.  If your school feels that 36 months is normal, put that on your DS 156.  But if you call them and they say that they made an error (which seems unlikely), then you should put whatever they tell you is the correct term of study.