Like any meal, the key to a good packed lunch is balance. A school lunch should be healthy, tasty, and easy enough that you (the parent) actually has time to pack it and 'thinking in threes' can help make this a reality.
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According to The Kitchn, this rule of threes applies to two things: nutritional content and number of items packed:
The rule of threes applies on a number of levels. First and foremost, we're talking the basics: protein, carbohydrates, and fat. But the rule of threes also applies to the actual items you pack. Three is just enough to provide variety (and variety is, after all, the spice of life) without being overly taxing in terms of assembly.
To become a master of efficient lunch-packing, check out the link below for tasty combinations (like ham roll-ups with cheese + banana + yogurt with pistachios) and more nutritional advice.