select isnull(isnull(a.name,b,name),c.name) as name
   ,a.学号
from a left join b on a.学号 = b.学号
left join c on a.学号 = c.学号

------解决方案--------------------
SQL code
select 
COALESCE(a.Name,b.Name,c.Name) as Name
COALESCE(a.学号,b.学号,c.学号) as 学号
from a full join b on a.Name=b.Name
full join c on c.Name=a.Name

------解决方案--------------------
case when

------解决方案--------------------
SQL code
select 
 case when
  a.name is not null then a.name 
 else
  case when b.name is not null then b.name else c.name end
 end
from
  tb

------解决方案--------------------
SQL code

select name=COALESCE(a.Name,b.Name,c.Name),a.学号
from a 
left join b on a.学号 = b.学号
left join c on a.学号 = c.学号