Peel the beets and slice them into thin wedges. Toss with 1 tablespoon olive oil, 1 teaspoon salt and 1/2 teaspoon pepper in a medium bowl to coat. Cut the sweet potatoes in half crosswise and cut each half into 8 wedges. Toss with 1 tablespoon olive oil, 1 teaspoon salt and 1/2 teaspoon pepper in a medium bowl to coat. Repeat the same process with the russet potatoes.
Arrange half of the beets on one-quarter of a baking sheet. Arrange half of the sweet potatoes on another one-quarter of the baking sheet, followed by half of the russet potatoes. Leave the last one-quarter of the baking sheet empty. Repeat with the remaining beets, sweet potatoes and russet potatoes on a second baking sheet, leaving the last one-quarter of the sheet empty. Cook the beets and potatoes, flipping them occasionally with tongs, until the potatoes are golden and softened, about 30 minutes.
Remove from the oven and add 4 fish fillets to each empty quarter of the baking sheets. Return to the oven and continue cooking the vegetables while you cook the fish according to the package instructions.
Meanwhile combine the mayonnaise, relish, mustard and lemon juice in a medium bowl to make a tartar sauce. Divide the fish fillets and vegetables among 4 plates, sprinkle with the parsley and serve with the tartar sauce and lemon wedges.