18 hours as a percentage of 3 days is :

1. $\dfrac{18}{3}\times 100\%$

2. $\dfrac{3}{18}\times 100\%$

3. $\dfrac{18}{3\times24}\times 100\%$

4. $\dfrac{3\times24}{18}\times 100\%$

Number of days = 3

Hours in 1 day = 24 hours

Total hours in 3 days = 3 x 24 hours

18 hours as a percentage of 3 days = $\dfrac{\text{Given hours}}{\text{Total hours}} \times 100$%

= $\dfrac{18}{3 \times 24} \times 100$%

Hence, option 3 is the correct option.