select a.姓名,b.班级,c.职务
from a
left join b on a.班级编号=b.班级编号
left join c on a.职务编号=c.职务编号

------解决方案--------------------
select A.姓名,B.班级名称 as 班级,C.职务名称 as 职务
from A inner join
B on B.班级编号=A.班级编号  
inner join C on C.职务编号=A.职务编号