Lightly grease the inside of the slow cooker. In a mixing bowl, stir together the melted butter, sugar, eggs, and vanilla until smooth.
Add the flour, cocoa powder, and salt to the wet mixture and stir until just combined. Fold in ¾ cup of the semisweet chocolate chips.
Pour the batter into the prepared slow cooker and spread it evenly. Sprinkle the remaining ¼ cup semisweet chocolate chips, the milk chocolate chips, and the white chocolate chips over the top.
Cover and cook on low for 2½ to 3 hours, until the edges look set but the center still has a slight give. Turn off the slow cooker and let the brownies rest for 15 minutes before serving.