Regarding this, is United States a nationality?
Normally speaking, people use "American" as the answer to nationality. However, all US passports show "United States of America" under Nationality.
Similarly, can an American citizen have 3 nationalities? U.S. law allows you to keep foreign citizenships even after you naturalize as a U.S. citizen. So you can become a U.S. citizen and keep your Canadian and Israeli citizenships. You can carry three passports. Canada and Israel are two of the many countries that disregard the renunciation in the U.S. naturalization oath.
Just so, what is the nationality of a person born in the United States?
A person born in the United States who is subject to the jurisdiction of the United States is a U.S. citizen at birth, to include a person born to a member of an Indian, Eskimo, Aleutian, or other aboriginal tribe.
Does your nationality change when you become a US citizen?
Yes, practically speaking. The U.S. government does not require naturalized U.S. citizens to relinquish citizenship in their country of origin. You may even lose your citizenship automatically in those countries upon becoming a U.S. citizen.