Preheat oven to 350˚F. Spray 2 - 8 ½ x 4 inch loaf pans with baking spray. Line the pans with parchment paper so that paper extends an inch or two over the top long edges of the pans.
Combine all three types of zest in a small bowl and mix lightly with the tines of a fork.
Combine lemon, lime and orange juice in a another small bowl. This juice will be used in the syrup/glaze, the icing and also in the bread. Divide the juice up like this; place 5 tablespoons of in a small sauce pan (for the syrup glaze). Place 3 tablespoons in a medium size bowl (for the icing). Set aside he remaining 4 tablespoons of juice for the bread batter.
For the bread, combine flour, baking powder, baking soda, salt and poppy seeds in a medium size bowl. Stir to combine and set aside.
Combine yogurt, milk and the 4 tablespoons of juice and vanilla. Stir or whisk together. Set aside.
Combine butter and sugar in the bowl of an electric mixer. Beat until light and fluffy, 2-3 minutes. Add eggs one at a time and mix well after each addition.
Add the flour mixture alternately with the yogurt/milk mixture, beginning and ending with the flour. Add zest (reserve just a bit of zest to garnish the loaves with after baking) and stir to combine.
Divide batter between the two prepared pans. Place pans on a sheet pan and bake for 50-60 minutes or until skewer or toothpick insert in the center of the loaves comes out clean.
Allow to cool slightly while preparing syrup.
For the syrup/glaze, add ⅓ cup sugar to the juice in the saucepan. Stir to combine and bring to a boil. Stir till sugar is dissolved, then remove from heat. Poke the top of the loaves all over with a skewer or toothpick. Slowly drizzle syrup over cake, allowing it to sink in before adding more. Use up all the syrup between the two loaves.
Allow cakes to cool completely in pans, then remove with the overhanging parchment paper.
For the icing, add 1 ½ cups of powdered sugar to the 3 tablespoons of juice in the medium bowl. Whisk together till smooth. Add more powdered sugar to make a thick, yet drizzle-able icing. Drizzle back and forth over tops of loaves allowing icing to drip down the sides. Garnish with a bit more zest, if desired.