We applied for a multi-entry visa and were given a doule entry with 3 month validity

Hi Marianna, it is normal. Sometimes even you apply for the 12-month multiple type, the officer only gives you a three-month double-entry type. There is no way to correct it. You can apply for the multiple entry type again, but I'm afraid you may be given the double-entry for a second time. So just use this double-entry type. When it expires, try to apply for the multiple-entry type.