This sounds pretty normal to me. My dd has days where she eats a few mouthfuls each meal and days when she can eat multiple meals, or beat my husband in quantity! She's always ben like this and she's now four. She's never been underweight except once when shed had a bad virus, and she put the weight back on rapidly after recovering.
My take on this is its a natural appetite cycle, it ebbs and falls depending on mood, activity, weather and who knows what else. If you're offering a good range of healthy foods at meal and snack times, she'll eat the quantity she needs.
I think we over think it and over regulate our meals as we get older, which can lead to weight problems in later life because I know i find it hard to eat less when I need less, because I'm so used to the three meals a day routine that you "have" to have. Even if my energy output is low and I'm not hungry, I seem to force dinner on myself. Why?