1. Preheat oven to 375 degrees.
2. Lightly grease a baking sheet with butter or non stick spray.
3. Mince the onion and garlic keep it aside, thaw the frozen chopped spinach.
4. Sauté onions and garlic in a small pan along with the oil, once done keep it aside until needed.
5. Mix the spinach, feta cheese, mozzarella cheese, Parmesan cheese, garlic, onion, nutmeg and salt in a bowl.
6. On a clean work surface arrange first sheet of phyllo dough and brush with melted butter. Cover the remaining phyllo dough with a damp towel.
7. Place a second sheet of phyllo on top and brush with butter, then place a third sheet on top of that and brush it with butter as well.
8. Cut the buttered phyllo lengthwise into four strips.
9. Place about 1 tablespoon of the spinach mixture on the bottom of each strip. Take the bottom right corner of the phyllo layers and fold the dough over the filling to make a triangle.
10. Fold the bottom left corner up to make another triangle. Continue folding until all the dough is folded.
11. Arrange the stuffed triangles, seam-side down, on the prepared baking sheet.
12. Lightly brush the triangle with butter. Repeat with the remaining phyllo dough and spinach filling.
13. Bake in the preheated oven until golden brown, about 20 minutes. Cool slightly before serving.
14. Serve the Spanakopita hot along with any dip you like.