Try this out:

DECLARE @Table table (
    Col1 char(1),
    Col2 varchar(6),
    Col3 smallint
)
;
insert @Table
    values
        ('A','before',100),
        ('A','after',10),
        ('A','after',80),
        ('B','before',280),
        ('B','before',20),
        ('B','after',250),
        ('C','before',150),
        ('C','after',220)
;
With groups (Col1, Col2, Col3) as (
    select Col1, Col2, sum(Col3) AS Col3
    from @Table
    group by Col1, Col2
)
select Col1, Col2, Amt
from (
    select '1' as Seq, Col1, Col2, Col3 as Amt from Groups where Col2 = 'before'
    union all
    select '2' as Seq, Col1, Col2, Col3 as Amt from Groups where Col2 = 'after'
    union all
    select '3' as Seq, B.Col1, 'Diff' as Col2, B.Col3 - A.Col3 as Amt
    from Groups B
    inner join Groups A on B.Col1 = A.Col1
    where B.Col2 = 'before'
    and A.Col2 = 'after'
) r
order by Col1, Seq