I am assuming that you want to wind it into a tight spiral, and that the 3 foot measurement is the diameter of the circle.

Then the area $\pi \frac{D^2}{4}$ is about 7.07 square feet or 1018 square inches, requiring 2036 inches or about 170 feet of cord.

I'm not giving an answer more precise than this because the diameter of the cord, probably your least accurate measurement, won't be accurate to better than a few percent. You had better allow 180 feet or so of cord if you want an accurate diameter, and be prepared to throw some away.

If 3 feet is the radius, not the diameter, then multiply area and cord length by 4.

Good Hunting!
RD