On the pH scale, a substance with a pH of 3 is best described as:

The pH scale runs from 0 to 14. A pH below 7 is acidic, exactly 7 is neutral (like pure water), and above 7 is basic (alkaline). A pH of 3 is well below 7, so the substance is an acid (and a fairly strong one, since lower numbers mean more acidic). Examples of acids in the low pH range include lemon juice and vinegar. A strong base would have a high pH (like 12–13). Remembering that 7 is neutral, lower is acidic, and higher is basic lets you classify substances by their pH quickly.