Final answer:
The phrase 'the product of a number and 3' as a variable expression is correctly represented by the expression 3n.
Step-by-step explanation:
The phrase 'the product of a number and 3' as a variable expression is represented by simply multiplying the variable by 3. The correct variable expression is 3n, where n represents the number. Therefore, the answer to which shows the phrase 'the product of a number and 3' as a variable expression is C. 3n, omitting the additional '+3' that was erroneously included in the option C provided.