use tempdb;
go

create table dbo.t (id int identity primary key, a int, b int, c int, n int, s varchar(100));
create index IX_t__a__b__c__n on dbo.t (a, b, c, n desc)
go

create trigger dbo.tr_t__iofinsert
on dbo.t
instead of insert
as
begin
 set nocount on;

 declare @g table (a int, b int, c int, base int, primary key (a, b, c));

 with s as
 (
  select top (1) with ties
   t.a, t.b, t.c, t.n
  from
   (select distinct a, b, c from inserted) i join
   dbo.t t with (index = IX_t__a__b__c__n, serializable, updlock) on t.a = i.a and t.b = i.b and t.c = i.c
  order by
   t.n desc
 )
 insert into dbo.t
  (a, b, c, n, s)
  select
   i.a, i.b, i.c, isnull(s.n, 0) + row_number() over (partition by i.a, i.b, i.c order by (select 1)), i.s
  from
   inserted i left join
   s on s.a = i.a and s.b = i.b and s.c = i.c
end;
go

insert into dbo.t
values
 (1, 1, 1, null, 'a'), (1, 1, 1, null, 'b'),
 (1, 1, 2, null, 'c'), (1, 1, 2, null, 'd')

select * from dbo.t order by a, b, c, n;
go

insert into dbo.t
values
 (1, 1, 1, null, 'e'), (1, 1, 1, null, 'f'),
 (1, 1, 2, null, 'g'), (1, 1, 2, null, 'h')

select * from dbo.t order by a, b, c, n;
go

drop table dbo.t;
go