Laymans take--there may be a different technical issue--but my layview of your question
It may not be cost effective to use counsel to chase $3000--so do more homework yourself if possible
My view is the annuity contract stands as written, firm is not a party to the divorce. If EX contracted to pay you, the firm needs to pay as per contract--ESPECIALLY if he contraced that way after divorce but even if he so contracted before divorce and let his contract stand. In a sense he gave you more than was required, its not a question of you taking more than was allowed .
Now there may be a separate issue under the retirement company terms and conditions that one cannot leave a benefit to anyone but a surving spouse unless there is a court order to that effect , a QDRO, and your EX made a mistake to leave it to a nonspouse --now if language like that is in the retirement plan you are probably up the creek,- go read plan cover to cover, line by line.