No, there shouldn't. If you added an 'are' it would make it an independent clause that could stand alone as sentence. For example:
Renewed fighting between Israel and Gaza in July and August left more than 2,100 Palestinians dead. About three-quarters of them are civilians.
As it is, the words 'about three-quarters of them civilians' refer back to the object 'more than 2,100 Palestinians', and add information about this object. It should not contain a verb. If you added a verb you'd be changing the grammar of the sentence.