In the Uk we use metric for weights etc, and this is one of the first things I change in all the X-cart Pro versions I've tested.
You will have to change several tpl files in addition to the 12/3 decimal in the database.
I posted some info on the following link in June about this.
http://forum.x-cart.com/showpost.php?p=127842&postcount=75
Note Also that
Code:
{ $product.weight|formatprice|default:$zero }
would need to be changed to
Code:
{ $product.weight|formatnumeric|default:$zero }
to produce 3 decimal places as in 2.756g.
I will search my system for the list of files and changes required and post back.