You are on the right path, all of these is accomplished via CSS/PT, so here is what you need to do:
put all group A DNs in parition lets call it GroupA_PT,
put all group B DNs in GroupB_PT
put all group C DNs in GroupC_PT,
create 3 Calling Search Spaces, each referencing it's own PT, and not the others, i.e.
GroupA_CSS, inlcudes GroupA_PT
Assign corresponding CSS to local devices, i.e. phone in group A would use GroupA_CSS.
That's it!!!
HTH,
Chris
