You have to cook 34 Squash Soup to obtain the first star, requiring 68 Onions, 102 Carrots, 170 Squash, 136 Potatoes and 8 hours and 30 minutes.
For the second star, you have to cook it 103 times, figuring in the + 100% Serving Reward from the first crop star, it requires a cumulative total of 138 Onions, 207 Carrots, 345 Squash, 276 Potatoes and 17 hours and 15 minutes. That is an additional 70 Onions, 105 Carrots, 175 Squash, 140 Potatoes on top of the first star, and an additional 8 hours and 45 minutes. Notice the cumulative numbers for the second star reflect cooking 104 since you can have to cook two recipes at time after the first recipe star reward.
The third star takes 206 Squash Soup, figuring in the + 100% Serving Reward from the first crop star, it requires a cumulative total of 240 Onions, 360 Carrots, 600 Squash, 480 Potatoes and 30 hours. From the perspective of the first star, you need an additional 172 Onions, 258 Carrots, 430 Squash, 344 Potatoes and 21 hours and 30 minutes. From the perspective of the second star, you need an additional 102 Onions, 153 Carrots, 255 Squash, 204 Potatoes and 12 hours and 45 minutes.