Preheat the oven to 375 degrees F.
In a large bowl, toss the blueberries, strawberries and raspberries with the lemon juice.
In a smaller bowl, blend the sugar, flour, cinnamon and nutmeg, then mix into the fruit and set aside.
For the topping, blend the oats, brown sugar, flour, cinnamon and salt. Slowly pour in the melted butter, mixing with a fork.
Pour the fruit into the pie shell and spread the crumb topping evenly over the entire pie.
Bake for 40 to 45 minutes, until the crumb topping is golden brown.
Dig in while it is hot!