If you miss the 3 hour window, you will have to reach out to the seller to ask if they will be able to cancel the order for you. This method is not guaranteed.
Otherwise, you will be unable to cancel the sale until 7 days after purchase. If this is the case, the seller you purchased from may be inactive and in fact never send the item.
You can cancel an offer at any point up until the seller accepting. Once the seller accepts the offer, the transaction is binding and you will not be able to cancel.