unique 'drag down' formatting

I need to create a database for a bunch of products, and after much organizing, have decided that a sequence following 000-000-000 will accommodate all of the products in an organized fashion.
Essentially, product category will be the first set of zeros, the subcategory the second set, and the sub-subcategory the third. Lets say we have red shoes, size 7. The sequence will run as 001-003-007 (1= shoes, 3= red, 7= size 7.) Yellow shoes will have a 001-004-007 sequence, etc.
Unfortunately, when I start typing the sequence into Excel, and try to drag down on the numbers, it changes from 001-003-007 to 001-003-008, when what I want to change is the middle set of numbers. Is there a way that will allow me to control what set of numbers increases? Or some other piece of software that will allow me to generate these numbers without having to do it manually?
Thanks!

So you want to achieve something like this after dragging down?

001-003-007
001-004-007
001-005-007

Yes. And then subsequently (probably under another drag down):

001-003-007
001-003-008
001-003-009