How To Design Product Kits

Hi,

I've run into a bit of a sticky design issue. We have products in
three categories which I will call 'A', 'B' and 'C'. We have "kits"
which contain three products, one from each category.

Below is some sample SQL to set things up, but I need to ensure that
each kit gets three products -- one from each category. Obviously,
this basic SQL doesn't allow that. Any suggestions? Do I need a
different schema design, or is there something else I should be
looking at?

Cheers,
Curtis

CREATE TABLE category (
id int identity primary key,
name varchar(30)
);

CREATE TABLE products (
id int identity primary key,
name varchar(30),
category_id int references category(id)
);

CREATE TABLE kits (
id int identity primary key,
name varchar(30)
);

CREATE TABLE kit_products (
kit_id int references kits(id),
product_id int references products(id)
);