If you need to know exactly three causes of tensions between the US and the USSR, you should probably consult your textbook or your notes from the class. This is because there are very many things that could be cited as causes of the tensions you are asking about. It is very hard to know which three factors you are supposed to cite without looking at your book or notes. I will give three causes, but they may not be the same ones mentioned in your book.
Overall, the main cause of the tensions between the US and the USSR was ideology. The USSR was a communist country and the US was a democracy with a capitalist system. These two systems were opposed to one another. Communists believed that their system would inevitably take over the world in time while Americans felt that the whole world would be better off if they were democratic. The two sides were therefore suspicious of one another.
Second, the Soviet Union wanted more of a buffer zone between it and the West. The Soviets feared invasions from Western Europe because they had been invaded at times in the past (most recently in WWII). Because of this, they took control of many of the countries of Eastern Europe rather than allowing those countries to choose whether they would be communist or democratic. This made the US feel the Soviets were bent on dominating all of Europe.
Third, the Soviets did not trust the US because of things that had happened in WWII. From June of 1941 until the Allied invasion of France in June of 1944, the Soviets were doing the vast majority of the fighting against the Germans. They pushed the US and the UK to open a major second front to ease the pressure on them but the Western Allies refused because they were not ready. This made the Soviets feel as if the Western Allies were using them to weaken Germany without any regard for what happened to the Soviet Union. This made them even more suspicious of the Americans.
All of these are reasons for the tensions between the two countries, but there are other reasons that could be cited. Please look in your textbook to see what answers you are supposed to give.