In the medium bowl, stir together flour, baking powder, and baking soda.
In the large bowl, cream together the butter, granulated sugar, and brown sugar. Beat eggs and vanilla extract into butter mixture.
Gradually stir in flour mixture. Stir in milk chocolate chips, semi-sweet chocolate chips, white chips, and nuts.
Drop by level scoop ( ¼-cupfuls), about 2 inches apart, onto ungreased baking sheets. Bake for 12 to 14 minutes or until light golden brown. Cookies will be puffy in the oven then will flatten as they cool. Allow cookies to cool for 2 minutes on baking sheets, then transfer to wire racks to cool completely.