Summing each of 3 groups of items in a table

[Dacker]

Items of 3 different groups are entered progresively, with their quantities, in a table. A formula is required to workout the total quantity in stock of each group for each month of the year.
I've uploaded a sample worksheet using Sendspace. Please download it using this link:
www[dot]sendspace[dot]com[slash]file[slash]04pgv1
Thanks in advance.