Put oven racks in upper and lower thirds of oven and preheat oven to 250°F.
Coarsely grate celery root into a bowl using the 1/3-inch-wide holes of a box grater.
Peel potatoes and coarsely grate into a large bowl. Add lemon juice and toss. Coarsely grate onions into same bowl.
Transfer to towel, then gather up corners to form a sack and twist tightly to wring out as much liquid as possible.
Return potatoes and onions to cleaned bowl and stir in celery root, flour, eggs, salt, pepper, and celery seeds until combined well.
Heat 1/3 inch oil in a 12-inch nonstick skillet over moderately high heat until hot but not smoking. Fill a 1/4-cup measure (not tightly packed) with latke mixture and carefully spoon it into skillet, then flatten to 3 inches in diameter with a slotted spatula. Form 3 more latkes in skillet, then fry until undersides are deep golden, 1 1/2 to 3 minutes. Turn over using 2 spatulas and fry until deep golden all over, 1 1/2 to 3 minutes more. (If latkes brown too quickly, lower heat to moderate.) Transfer to paper towels to drain briefly. Keep warm in 1 layer on a metal rack set in a shallow baking pan in oven. Make more latkes in same manner. Use a second rack and baking pan to keep last batches warm.