Start by draining the mandarin oranges well so they don't water down the salad later on. Chop the fresh pineapple into bite-sized chunks and slice the bananas into thin rounds. Toss the banana slices with the lemon juice right away so they stay bright and don't turn brown.
In a small bowl, whisk the vanilla yogurt until it's smooth and lump-free. Gently fold in the whipped topping using your spatula, turning the bowl as you go rather than stirring hard. You want the mixture light and airy, not deflated or runny.
Add the mandarin oranges, pineapple, and bananas into your large mixing bowl. Pour the creamy dressing over the fruit and fold everything together carefully with the spatula. Stop as soon as the fruit is evenly coated so it doesn't turn mushy.
Cover the bowl with plastic wrap or a lid and place it in the refrigerator. Let it chill for at least 30 minutes so the flavors have time to blend together. This step also firms the dressing up slightly, which makes serving much neater.
Right before you're ready to serve, scatter the candy corn candies and mini marshmallows over the top. I always wait until the last minute for this part because the candy corn can get sticky and start to melt if it sits too long. A light touch here keeps the presentation clean.
Give the salad one last gentle toss, or leave the candy corn candies resting on top for looks. Spoon it into bowls or cups and watch it disappear within minutes at any Halloween gathering. It's just as good eaten straight from the fridge as a quick snack.