Add celery root to a large pot of boiling salted water. Reduce heat to medium-low and simmer until tender, 11-13 minutes. Using a slotted spoon, transfer celery root to a large bowl. Return water to a boil; repeat with kohlrabi, then potatoes, cooking each separately until tender, 14-16 minutes for kohlrabi and 10-12 minutes for potatoes; add to bowl with celery root. Meanwhile, bring apple and 2 tablespoons water to a boil in a small saucepan. Cover and cook over medium-high heat, stirring occasionally, until apple falls apart, 6-8 minutes, adding water by tablespoonfuls if dry. Working in batches, pass celery root, kohlrabi, potatoes, and apple through a potato ricer into a large bowl. Pass mixture through ricer again if a smoother texture is desired. Stir in butter. Season with salt and pepper. DO AHEAD: Can be made 1 day ahead. Transfer to a microwave-safe bowl, cover, and chill. Rewarm in microwave in 30-second intervals until heated through. Transfer to a large serving bowl. Garnish with chervil sprigs.