What are bittering hops and aroma and flavoring hops? Do I need to add three different kinds of hops?

Hops behave in different ways depending on when they are added to the wort. The hops added at the beginning of the boil produce bitterness, the aroma and flavor they possess will have boiled off before the end. Hops added later in the boil produce flavor, they don't have enough time to produce much bitterness, and their aroma will have boiled away. Finally, hops added at the end don't have enough time to produce much bitterness or flavor, but they will give aroma to the mix. To sum up, it is the timing of when the hops are added that produces bitterness, flavor, and aroma. Of course, individual hops each have their own characteristics and this makes the subject of hopping complex.