Heat oven to 350º F. Line a rimmed baking sheet with parchment.
Combine the maple syrup, sugar, and butter in a medium pot. Bring to a simmer over medium heat. Add the pepitas, pecans, walnuts, cayenne, and ¼ teaspoon salt. Simmer, stirring, until thickened, 2 to 4 minutes.
Transfer the nuts and liquid to the prepared baking sheet and spread in a single layer. Bake, stirring once, until the nuts are golden and sugar is crystallized, 7 to 10 minutes. Stir once more, then let cool; break into clumps.