Hello,

Apparently, you can apply for your Spanish nationality providing with your Cuban nationality, however, the civil registry may remind you that you entered Spain as an American citizen and could have issues to grant a third nationality to someone who holds two already. Rest assured you will be forced to renounce to your American nationality, at least before the Spanish authorities eyes.

If you are Cuban by birth, that is, you were born in Cuba, your Cuban passport will be accepted when applying for Spanish nationality within two years.

Regards,