Preheat oven to 375 degrees.
Combine the rhubarb, strawberries, apple, sugar, lemon juice, cornstarch and a pinch of salt in a 9″ x 13″ baking pan. Toss gently to combine.
In a large bowl, combine the oats, flour, and brown sugar.
Add the butter to the flour mixture and cut in the butter pieces with a pastry cutter, a large fork, or your fingers until the mixture forms coarse crumbs.
Sprinkle the topping over the strawberry rhubarb filling.
Bake at 375 degrees for approximately 40 minutes until the filling is bubbly and the topping is crisp and browned.