Pour flour, sugar, salt, milk, eggs, and 3 tablespoons melted butter into a blender.
Puree until mixture is smooth, about 30 seconds. Let batter sit for 15 minutes before you use it.
Heat a 12-inch nonstick skillet on medium heat. Lightly coat with additional butter. Add scant 1/3 cup batter and swirl to completely cover bottom of skillet. Cook until underside of crepe is golden brown, 2-3 minutes.
Loosen edge of crepe with a spatula, then with your fingertips, quickly flip. Cook 1 minute more. Slide crepe out of skillet and repeat with remaining batter.
Spread 1 1/2 tablespoons of nutella over each crepe and add a sprinkling of berries. Fold each crepe into a triangle. Top with a dollop of whipped cream and serve immediately. Garnish with mint if desired.
Make Ahead: The crepe batter can be made earlier in the day and stored in the fridge. Mix it well before use.