Be prepared for a delay in closing. You have modified your credit situation which may concern the mortgage lender, you may have also deceived the store if you didn't disclose the new mortgage you were in the process of getting. You need to address this with your real estate agent.
One scenario is they will discover the new credit card, and ask for you to write a letter describing the situation. They may need to re-approve the mortgage based on the new numbers, they will also be concerned that the $3,000 purchase will impact your ability to make the down payment.
They might not discover the new credit card; but at the closings I have been involved in this potential gap was covered by them asking if there were changes in bank account balances, loans and credit cards. You will either have to disclose it at closing, or lie about it. I suggest you tell the truth.
The large purchase just before closing, even if a new credit card was not involved, could cause the lender to be concerned. It would have impacted the other debt number, or reduced the amount in savings.