select ID,NAM,max(N1) N1,max(N2) N2,max(N3) N3
from (你union all后的结果) t
group by t.ID,t.NAM;

------解决方案--------------------

SQL code

--> 测试数据：@表1
declare @表1 table([ID] varchar(3),[NAM] varchar(3),[N1] int,[N2] int,[N3] int)
insert @表1
select '001','AAA',80,0,0

--> 测试数据：@表2
declare @表2 table([ID] varchar(3),[NAM] varchar(3),[N1] int,[N2] int,[N3] int)
insert @表2
select '001','AAA',0,50,0

--> 测试数据：@表3
declare @表3 table([ID] varchar(3),[NAM] varchar(3),[N1] int,[N2] int,[N3] int)
insert @表3
select '001','AAA',0,0,30

SELECT 
    ID,NAM,MAX(N1) N1,MAX(N2) N2,MAX(N3) N3 
FROM 
(
    SELECT * FROM @表1 UNION ALL
    SELECT * FROM @表2 UNION ALL
    SELECT * FROM @表3
)T GROUP BY ID,NAM
/*
ID   NAM  N1          N2          N3
---- ---- ----------- ----------- -----------
001  AAA  80          50          30
*/