I handle real estate broker matters and am familiar with brokerage law.
It is unlikely that a real estate broker would have to refund a commission because three years after the closing a buyer defaulted on a seller held mortgage.
I assume that the broker did not sign as a guarantor for the buyer.
Unless you can show that the broker made affirmative misrepresentations of fact concerning the buyer's ability to pay or had actual knowledge of the buyer's inability to pay - which the broker did not disclose - the commission will not be returned.
If this answer was helpful, please mark it as helpful or as a best answer. This answer is for general education purposes only. It neither creates an attorney-client relationship nor provides legal guidance or advice. The answer is based on the limited information provided and the answer might be different had additional information been provided. You should consult an attorney.
Your question is not detailed enough to give a detailed answer but, absent something in the contract which states otherwise or something in the facts like a misrepresentation, a brokers commission is not returnable after closing.
You should hire an attorney that specializes in real estate to go over the contract and advise you properly.