I had this problem last week (and a few times before already).
You can contact customer support and tell them the number of the order you want to keep and the order numbers you want them to cancel, they’ll do that for you if it was accidental multiple orders.
Sadly, it still will lower your completion rate percentage.
You should, after all, message the buyer to ask about it, though, in case they actually wanted to order 3x and just didn’t have the time to provide the requirements for the other two orders but keep in mind that if they won’t respond timely, it may affect your delivered on rate percentage and when an order goes very late, the client will get the option to cancel, which again will lead to an automated 1* review if they do so.
So, the best option depends on how much time you have and on whether the buyer will react or not; with a cancellation, you’re on the safe side regarding the order going late, but it’s not ideal either, obviously.