Do you need a return flight if you have a 3 year visa to stay in America?

No, because you cannot have a ticket issued until 330 days before departure.

From the US Dept. of State’s website: Please be aware, a visa does not guarantee entry to the U.S. Additionally, the visa expiration date shown on your visa does not reflect how long you are authorized to stay within the U.S. Entry and the length of authorized stay within the U.S. are determined by the Customs and Border Protection (CBP) Officer at the port-of-entry each time you travel.

It is important to note that there are circumstances which can serve to void or cancel the period of visa validity. If you overstay the end date of your authorized stay, as provided by the CBP officer at a port-of-entry, or United States Citizenship and Immigration Services (USCIS), your visa will automatically void or cancel unless;

You have filed an application in a timely manner for an extension of stay or a change of status;
That application is pending and not frivolous;
If you have applied for adjustment of status to become a permanent resident alien (“green card” holder), you should contact USCIS regarding obtaining Advance Parole before leaving the U.S.

Admission to the U.S. and your Duration of Stay
Upon arriving at a port-of-entry and when admitted, a CBP officer places a small white card, Arrival-Departure Record, Form I-94 or a small green card, Form I-94W for VWP travelers, in your passport. This card is very important as it shows permission to be in the U.S. On this card, the CBP officer records either a date or duration of status (D/S) in the lower right hand corner. If your I-94 or I-94W contains a specific date that signifies the date you must exit the U.S. Some students, exchange program participants, and temporary workers (e.g., foreign diplomats) will be admitted for D/S. If you have D/S on your Form I-94, you may remain in the U.S. as long as you continue your course of studies, remain in your exchange program, or qualifying employment. The date or D/S notation, shown on your Arrival-Departure Record, I-94 or I-94W is the official record of your authorized length of stay in the U.S. You cannot use the visa expiration date in determining or referring to your permitted length of stay in the U.S.

http://travel.state.gov/visa/temp/info/info_1298.html

Does the visa allow you to live in the U.S. for 3 years, or is it valid for 3 years? Those are very different things.

As @Tropical_Willie said, you can’t buy a plane ticket that far ahead, so your best bet is to have proof that you intend to return to your home country.