All banks in Spain will cash your travelers checks but they too will charge a commission for doing so. I really don't know why anyone would use these anymore as they 'get you' coming and going AND you have to adhere to the banking hours/days. You really won't find it easy, if even possible, to pay for things with travelers checks.
Most everyone now uses their ATM ("cajero" / automatic bank machine) cards or bank cards (not to be confused with credit cards) and then they only pay a small-ish conversion charge (maybe less than the 3% you're paying for the travelers checks - see your bank for details).
Saludos, MadridMan
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