In a non-stick frying pan, heat 1 tsp of peanut oil over a medium heat and gently fry the garlic. Add the pork and stir until it browns, then cover the pan and cook for 5 minutes. Transfer the pork to a mixing bowl and set aside to cool.
Chop the prawns and add to the cooled pork with the chopped chicken and bean sprouts.
In a small jug, whisk together the fish sauce, brown sugar, lime juice and sweet chilli sauce, then pour over the pork mixture. Add half the fried shallots, the diced watermelon, fresh mint and coriander. Toss together and serve scattered with the remaining shallots.