Meaning of 的 in 只有三个得满分的
听说只有三个(考生)得了满分 -- I heard that only three (examinees) got full marks

听说只有三个得了满分的(考生)--  I heard that there are only three (examinees) who got full marks

The object 考生 was omitted

考生 = examinee

得了满分的考生 = examinee who got full mark 

'得了满分的' is an adjectival phrase that describes '考生'