Hi @Wendolene thanks for your post, although we're sorry to hear of your confusion here surrounding the switchover.
You are correct in suggesting that you'll not need a new Hub, if you have a Hub 3 already. It's possible that this letter would have been generated, before you switched out any older Hub to your Hub 3.
Have you had a Hub change within the last few months? If so, this would explain things - please let us know however, if you need any further help.
Many thanks
Tom_W