Soak the onions in a bowl of ice water to mellow their sharpness and set aside. Strain and rinse the black, kidney and pinto beans.
Add 3 tablespoons of the oil, the tomato paste, chili powder, cumin and oregano to a medium skillet. Turn the heat to medium-low and cook, stirring frequently with a wooden spoon or heatproof spatula, until the tomato paste and oil have come together as a dark brown paste, 6 to 8 minutes. Stir in the garlic and cook for 1 minute more. Turn off the heat, and stir in the remaining 5 tablespoons oil. Let cool for 25 minutes so the oil can absorb the flavors of the tomato-paste mixture.
Pour the oil mixture into a fine-mesh strainer set over a liquid measuring cup with a spout. Do not press down on the tomato-paste mixture, just let the oil drain until there is at least 1/4 cup of it in the measuring cup, 5 to 7 minutes. Discard the tomato-paste mixture. Put the pickled jalapeno juice and 1 teaspoon salt into a large mixing bowl, and slowly whisk in the flavored oil.
Strain the onions, and add them to the bowl with the dressing. Add the jalapenos, all the beans and the red and yellow bell peppers. Stir to combine, and add salt to taste. Cover with plastic wrap, and refrigerate for at least 30 minutes or up to overnight. Serve with a dollop of sour cream, and garnish with the sliced scallions.