To find the rate, we note that the cost of 3 pounds of potatoes is $12.
The unit rate is calculated by dividing the total cost by the total weight:
\[ \text{Unit Rate} = \frac{\text{Total Cost}}{\text{Total Weight}} = \frac{12 \text{ dollars}}{3 \text{ pounds}} = 4 \text{ dollars per pound} \]
So, the rate is $12 for 3 pounds, and the unit rate is $4 per pound.