Saute the spinach in a large skillet until wilted, then set it aside to cool. When cool enough to handle, squeeze it gently to remove excess water, then chop it.
Melt 2 tablespoons of butter in the same skillet and saute the diced shallots until almost soft, then add the bell pepper and continue cooking until the veggies are soft.
Mix the ricotta, mozzarella, feta, squeezed and chopped spinach, and shallot-pepper mixture in a large bowl until well-mixed. Season with salt and pepper.
Fold in the crab gently to not break up any big lumps.
Preheat your oven to 375-F and line a baking sheet with parchment paper.
Lay one sheet of phyllo pastry on a clean work surface and brush it with melted butter. It doesn't have to be completely covered with butter.
Top the buttered sheet of phyllo with another sheet, and brush it as well. Repeat with one more buttered layer, then a final sheet of phyllo with no butter. You should have 4 layers of phyllo, 3 of them buttered.
Turn the sheets so that the longest side is facing you. Cut the sheets in half, creating 2 equal portions. Now cut those into thirds, creating 6 even-sized strips of phyllo.
Place about 1.5-2 tablespoons of filling near one end of the phyllo strip, then fold the bottom left corner up over the filling toward the long right edge of the dough strip to create a triangle.
Fold the bottom right corner of the phyllo straight up, creating another triangle. Now take the bottom right corner and fold it up and to the left, lining it up with the long left edge of the phyllo strip, creating another triangle.
Continue folding it like a flag until it's fully folded. Brush the top with butter and lay it seam-side down on a parchment paper-lined baking sheet.
Repeat with the remaining sheets of phyllo and filling.
Bake the phyllo triangles at 375-F for about 15 minutes or until they're golden brown on top.
Cool and serve!