Whisk the pudding mix, milk and mango puree together in a large bowl for 2 minutes to fully blend and thicken. Pour the mixture into the graham-cracker crust, filling the crust just shy of the top. Spread evenly with an offset spatula, then refrigerate until chilled and set, about 2 hours.
Meanwhile, prepare the fruit: Cut off the ends of the kiwis, and then peel off the skin by slipping a spoon under the skin and turning the fruit so the spoon separates the skin from the flesh. Slice the kiwis into 1/8-inch to 1/4-inch thick rounds. Peel the mango, then trim the bottom end to flatten slightly. On the wider side of the mango, work inwards cutting thin slices about 1/8-inch to 1/4-inch thick until you reach the pit; repeat on the other side. Next, peel and cut the pitaya into 1/8-inch-thick to 1/4-inch-thick slices. Using a 1 1/2-inch diamond-shaped cutter and a paring knife, cut out about 20 kiwi diamonds, 20 mango diamonds and about 10 pitaya diamonds (this will give you a few pieces more than you'll need). If you don't have a diamond-shaped cutter, use a paring knife to cut out diamonds that are each about 1 1/2 inches long.
Once all of your fruit is cut, remove the tart from fridge and begin decorating: Using a small offset spatula to help place the fruit, start in the center of the tart and make a large diamond shape with 9 of the pitaya diamonds. Then make a border around the pitaya with 16 mango diamonds. Then place the kiwi diamonds around the mango, covering the tart all the way to the edges; you will need to trim off parts of the remaining kiwi diamonds to fit along the crust.