Preheat the oven to 425ºF (220ºC).
Add the rhubarb, strawberries, apple, sugar, lemon juice, and flour to a deep bowl. Stir to combine; set aside.
Roll out one piecrust into a 10-inch circle. Gently fit it into a 9-inch pie plate.
Spread the filling into the crust.
Sprinkle the crumb topping over the filling.
Roll the remaining piecrust into a 9-inch circle. Cut the dough into ½ to ¾-inch strips, and arrange on top in a lattice pattern. Trim the edges, and crimp the crust or flute the edges, as desired.
Bake for 15 minutes. Reduce oven temperature to 350ºF. Bake until the crust is a deep golden color, and the juices are bubbling from the pie, 30 to 40 minutes. Let cool completely before cutting.