Preheat the oven to 325oF.
Line two 12-hole muffin / cupcake pans with cupcake liner.
Sift together the flour, baking powder and salt. Add 1-1/2 cups of sugar to the flour mixture and stir well.
Make a well in the center of the flour mixture and pour in the oil, egg yolks and combined juices of the lemon and oranges. Add the lemon and orange zest as well. Mix until blended.
The batter will be heavy at this point.
Beat the egg whites until soft peaks form. Continue beating, adding the 3 tbsps. of sugar little by little, until stiff peaks form (see stages in beating egg whites).
Add half of the egg whites to the egg yolk-flour mixture. Mix — gently so as not to break the air bubbles in the egg whites — with a whisk (or with a rubber scraper using a cut and fold motion) to combine.
Pour the batter on the remaining egg whites. Mix gently until smooth.
Pour about 1/4 cup of batter into each hole. Bake at 325F for 30 minutes.
Serve the cupcakes warm and plain -- they're delicious while warm.
Or cool the lemon-orange chiffon cupcakes completely and pile icing or frosting on top. Another option is whipped cream.
Or, if you prefer simplicity, sprinkle with sugar and serve.
Serve the lemon-orange chiffon cupcakes with coffee, tea or cold juice.