Toss the pepitas with the olive oil, 1/4 teaspoon of the chili powder and 1/4 teaspoon salt. Add to a small skillet and heat over medium heat, stirring frequently, until toasted and turning brown in some spots, 5 to 7 minutes.
Use a melon baller to scoop the watermelon, honeydew and cantaloupe into a large bowl. Mix the lime zest and juice, honey, remaining 1/2 teaspoon chili powder and 1/4 teaspoon salt in a medium bowl. Pour the dressing over the melon and toss until well coated. Fold in the pepitas and serve immediately.