The boys who （are）playing football are the Class 3 student.
那些正在踢足球的男孩们是三班的学生.
定语从句,who are playing football 修饰 the boys
主语boys为复数,用are.
可以将此句分成两个句子,就可以看出来了：
The boys are playing football；
The boys are the Class 3 student.