My son is older now (5) but that wouldn't be enough for him and nor would it have been at 3. He is very tall for his age and skinny but was born at 9lb 6oz so has always been on higher centiles.
On a typical day, he would have:
Breakfast :
Weetabix with semi skimmed milk, cup of milk and either toast/mini malt bread/Apple.
Fruit and milk at school snack time (mid morning)
School lunch - obviously varies but things like sausages with mash and beans, cucumber sticks on the side and then fruit/yogurt for dessert.
School milk in the afternoon.
Then when he gets home he has a snack - cheese/fruit/crackers etc.
Then dinner with us - typically something like cottage pie/lamb chops with veg/potatoes etc. Followed by yogurt.
He then has a banana and milk at bedtime!
Looking at it that looks loads - decent portions too
but as I say, he is very tall and a perfect weight for his height. He's very active and just seems to need a lot of food! Always has.
I would maybe mention that he seems very hungry or send a packed lunch. Can have weetabix for breakfast - might be more filling?