When planning a 3-day trip, choosing the right backpack size is crucial for comfort and convenience. In my experience, a 30-40 liter backpack is ideal for packing essentials like clothes, toiletries, and some basic tools. It's important to remember that the size may vary depending on personal packing habits and the expected weather conditions. Ultimately, it's best to find a balance between having enough space for your belongings and not overpacking to avoid unnecessary weight. Remember to test out different backpack sizes to find the perfect fit for your individual needs and preferences.