National Express East Coast don't accept GroupSave.
National Express East Coast, GROUPSAVE NOT ACCEPTED - ADVISE CUSTOMER NOT TO USE GROUPSAVE TICKET ON ANY TRAINS OPERATED BY THIS TOC
The OP would be better contacting NXEC group travel,
http://www.nationalexpresseastcoast.com/group, as they offer discounts and advice for groups of 10 or more.
Edit: just moved post into more appropriate location
--- old post above --- --- new post below ---
It's irrelevant in this case anyway as NXEC don't accept GroupSave, but why did you only look at GroupSave 3? GroupSave 4 for 12 people, and GroupSave 3 for 3 people would have been far cheaper.