Wrong! A certificate of Live birth IS the official certificate. Our oldest son has one from the US state department (he was born overseas) and it says exactly that.
This is no longer true. Fewer and fewer shops are accepting dollars, especially for small purchases and if they do the exchange rate will be really bad. Many businesses are reluctant to accept cash at all and prefer payment by credit or debit card. Euros are definitely the way to go and/or a card with no foreign transaction fees.
The way you wrote it makes it sound like you cannot use it for lunch and dinner on the same day at all. As long as the total number of meals is three you can use them for any meal.