Rinse grated potatoes in water and then press between paper towels to remove water; set aside. Roll grated aucchini in paper towel and press well to remove any excess moisture. Combine potatoes, zucchini, carrots, onion, eggs, flour, salt and pepper, to taste, in another large bowl. Heat a thin layer of oil in a non stick skillet over medium high heat. Drop vegetable mixture by heaping tablespoons (15 mL) into the oil, flatten slightly, and fry on both sides until golden brown. Using a slotted spatula, transfer rosti to a paper towel-lined tray to remove excess oil. Repeat with remaining vegetable misture, adding oil as needed.
Serve rosti warm with a dollop of herbed cream cheese or red pepper jelly, if desired.