Using Cte it is possible
DECLARE @table TABLE(Keywords varchar(100))
insert into @table values('SER_ELECTR_003')
;with cte as (
select Keywords,substring(Keywords,0,charindex('_',Keywords))Key1
,SUBSTRING(Keywords,charindex('_',Keywords)+1,LEN(Keywords)-charindex('_',Keywords))Key2
from @table
)
select Keywords,Key1,substring(Key2,0,charindex('_',Key2))Key2
,SUBSTRING(Key2,charindex('_',Key2)+1,LEN(Key2)-charindex('_',Key2))Key3
from cte