Table Formating in Numbers 3.0
This can be accomplished manually.

Changing cell border line weight


Select the desired row(s) [Multiple rows can be selected using the ⌘
+Click method.]
Select Cell tab in the Format inspector pane
Select the desired border [i.e.: Top, bottom, left, right, all]
Change the line style and thickness (weight)




Repeat for each desired row.  The Copy Style / Paste Style buttons can speed up your work as well.