What are three differences between RNA and DNA?

Answer

Three key differences are: (1) RNA is usually single-stranded, while DNA is double-stranded. (2) RNA contains the sugar ribose, while DNA contains deoxyribose. (3) RNA uses the base uracil (U), while DNA uses thymine (T).

Explanation

What you are being asked to compare

You need three clear, standard textbook contrasts between RNA and DNA. The easiest way is to compare structure (strands), sugar type, and one nitrogen base.

Difference 1: Strand structure

DNA: typically a double-stranded helix.
RNA: typically single-stranded, although it can fold back on itself to make complex shapes.

Difference 2: The sugar in the backbone

DNA contains deoxyribose, which has one less oxygen than ribose.
RNA contains ribose.

A quick memory cue: deoxy- means "missing oxygen".

Difference 3: One base is swapped

DNA bases: $A, T, C, G$ (adenine, thymine, cytosine, guanine)
RNA bases: $A, U, C, G$ (adenine, uracil, cytosine, guanine)

So RNA uses uracil (U) in places where DNA uses thymine (T).

Extra note (not required, but common)

DNA is mainly for long-term information storage, while RNA often helps carry out protein synthesis (for example, mRNA, tRNA, and rRNA).