Вид	            Модель	Количество
Компьютер	      a1	21
Компьютер	      a2	53
Компьютер	     a3	48
Принтер	             b1	26
Принтер	             b2	19
Принтер	             b3	45
Принтер	             b4	53
Принтер	             b5	17

select top  1 *
....
order by ceiling(Row_number() OVER (PARTITION BY вид ORDER BY количество DESC)/3.0)

declare @t table (prod varchar(100), mod varchar(100), qty int);
insert into @t values
('computer',	      'a1',	21),
('computer',	      'a2',	53),
('computer',	     'a3',	48),
('printer',             'b1',	26),
('printer',             'b2',	19),
('printer',             'b3',	45),
('printer',             'b4',	53),
('printer',             'b5',	17);

with cte as 
(
select distinct prod 
from @t
)

select a.*
from cte c cross apply (select top 3 with ties *
                      from @t t
					  where t.prod = c.prod 
					  order by qty desc)a;
---
prod	mod	qty
computer	a2	53
computer	a3	48
computer	a1	21
printer	b4	53
printer	b3	45
printer	b1	26