Query To Find The Top 3 In Each Type

Hi

I need help in finding the query which will provide the following resultset from the below table..

Table :
create table product_stocks(product_id int , product_type varchar(20) , no_of_units int)

Data:
insert into product_stocks values(1,'A',30)
insert into product_stocks values(2,'A',70)
insert into product_stocks values(3,'A',60)
insert into product_stocks values(4,'A',40)
insert into product_stocks values(1,'B',90)
insert into product_stocks values(2,'B',60)
insert into product_stocks values(3,'B',70)
insert into product_stocks values(4,'B',40)
insert into product_stocks values(1,'C',40)
insert into product_stocks values(2,'C',50)
insert into product_stocks values(3,'C',80)
insert into product_stocks values(4,'C',90)

Result Set:
product_type product_id no_of_units
--------------- ------------- --------------
A 2 70
A 3 60
A 4 40
B 1 90
B 3 70
B 2 60
C 4 90
C 3 80
C 2 50

i.e The result set gives the top 3 products in each product_type based on the no_of_units.

thanks