Heat a large skillet over medium-high. Add 1 bag spinach; cook, turning with tongs, until just starting to wilt, about 2 minutes. Repeat with the second bag. Stir in the third bag and cook until completely wilted, about 2 minutes more. Transfer spinach to a colander; let cool 10 minutes, then squeeze out the liquid and coarsely chop. In a medium saucepan, melt the butter over medium. Add the garlic; stir until just starting to brown, about 2 minutes. Add the flour. Cook, stirring often, until pale golden, about 3 minutes. Whisk in the milk, nutmeg, 1/8 tsp. salt and 1/8 tsp. pepper. Cook, whisking constantly, until thickened and just starting to boil, about 3 minutes. Add the cheese; whisk until melted, about 30 seconds. Stir in the spinach. Cook, stirring, until the spinach is heated through and evenly coated with the sauce, about 1 minute.