1. In a small bowl, combine the gelatin and 1/2 ounce milk. Whisk with a fork to ensure no lumps remain. Set aside.
2. In a small pot, combine the remaining milk, cream, sugar and salt. Place over medium low heat, stirring occasionally until the sugar has dissolved and the dairy begins to bubble.
3. Meanwhile, place the three chocolates into three separate bowls. Microwave each in a few 8 second bursts until fully melted. Set aside.
4. Once the pot of milk and cream has begun to simmer, remove from heat and whisk in gelatin.
5. Carefully pour about once once of the base into the bowl of dark chocolate. Whisk gently with a fork until homogenous. Pour in another 6 ounces of the base and whisk until smooth (7 ounces total). Set aside.
6. Likewise, pour about an ounce of the base into the bowl of milk chocolate and whisk until smooth. Add in another 5 ounces (6 ounces total). Set aside.
7. Pour in an ounce of base into the white chocolate. Whisk until smooth. Pour in another 3 ounces of base and whisk until smooth. If any panna cotta base is left over in the pot, divide it evenly between the three mixtures and whisk gently to combine.
8. Set out six 3 ounce juice glasses or plastic drinking cups (like Dixie) and grease lightly with pan spray. Alternately, the panna cotta can be prepared in champagne flutes, which do not need to be greased.
9. Pour 3/4 ounce of the white chocolate base into each cup. Refrigerate for 45 minutes, or until the panna cotta has set up enough to no longer be tacky to the touch.
10. When the first layer has set, whisk the milk chocolate mixture vigorously to break up any lumps or skin that has formed. Pour an ounce of milk chocolate base into each cup. Refrigerate an hour until likewise set. As before, whisk the dark chocolate layer vigorously to break up any lumps or skin. Pour 1 1/4 ounces of dark chocolate panna cotta base into each cup.
11. Refrigerate another 4 hours to ensure a full set before unmolding. The finished panna cotta will keep, refrigerated, up to ten days.
12. To unmold, invert the cups over a dessert plate. Use your index finger to gently pull the panna cotta away from the side of the glass; once a little air slips in between the panna cotta and the glass, the entire panna cotta should slide out easily. Note: If using champagne flutes, unmolding obviously not required.