create trigger upd_table on table
for update
as

create table #dropme (
    blah int identity(1,1),
    username nvarchar(??)
)

declare
    @blah int,
    @maxblah int,
    @sql nvarchar(300)

insert #dropme
select
    t.username
from
    [table] t
where
    exists (
        select
            1
        from
            deleted
        where
            username = t.username)
group by t.username
having count(t.username) < 3

select
    @blah = 1,
    @maxblah = max(blah)
from
    #dropme

while @blah <= @maxblah
begin
    select
        @sql = N'exec master..sp_droplogin '''+username+'''',
        @blah = @blah + 1
    from
        #dropme
    where
        blah = @blah

    exec master..sp_executesql @sql
end

go