Preheat oven to 350F (180C). Peel and core apples. Slice into thin wedges. Place in a large bowl. Hull strawberries, then cut into quarters and place in bowl with apples. If using frozen rhubarb, place in a sieve and rinse under cold water to remove ice crystals. Slice larger pieces in half. Cut fresh rhubarb into 1-inch (2.5-cm) pieces. Place rhubarb in bowl with other fruit. In a small bowl, stir 3 tbsp (45 mL) each sugar and flour with 1/2 tsp (2 mL) cinnamon. Sprinkle over fruit and toss to evenly coat. Turn into an 8-inch (2-L) square baking dish. Gently press down to even out surface.
For topping, in a medium-size bowl, stir oats with flour, brown sugar, cinnamon and nutmeg. Slowly drizzle in butter, stirring in with a fork. Evenly scatter overtop fruit.
Bake, uncovered, in centre of preheated 350F (180C) oven until fruit starts to bubble around edges and a knife tip inserted in centre of crisp comes out hot, about 1 hour. Serve warm or at room temperature with a scoop of light vanilla ice cream or a dollop of yogurt. Leftovers will keep well, covered and refrigerated, 2 to 3 days.