Translation:They saw three airplanes above themselves.
"They saw three airplanes above them" is also accepted. The sentence using "themselves" may have been selected as the primary translation to emphasize the meaning of sebou. The English sentence that you suggested can also be interpreted as some group of people ("they") saw three airplanes above "them," where "them" refers to some other group of people. But with sebou, that meaning is not possible.