What three components make up a nucleotide?
A nucleotide is a building block of DNA. It is a three-part structure that is composed of a nitrogenous base, a "pentose" sugar, and a phosphate group.
In class, I prefer calling the pentose sugar a five-carbon sugar because it helps students remember that the sugar in a DNA nucleotide contains five carbons. In DNA this five-carbon sugar is a deoxyribose sugar; and it is a ribose sugar in RNA.
The nitrogenous base could be one of four possible bases. It could be cytosine, thymine, adenine, or guanine. Those four bases can also be subdivided into two groups. Adenine and guanine are purines, while cytosine and thymine are pyrimidines. Adenine pairs with thymine and guanine pairs with cytosine. It's the order of those bases that give a DNA strand its unique signature.
As for how the sugar, base, and phosphate are put together, the sugar is in the middle of the nucleotide with the phosphate and nitrogen base sticking off either side.