if i have 3 routers
A has a mutlipoint to point connection with both B & C
should the 3 interfaces be in the same subnet ?!
or A-C in subnet and A-B in a nother subnet ?!
As for as I know,
Yes.. for point to Multipoint interfaces, all of Hub and Spokes have to be on the same Subnet..
for point to point each hub and spoke pair should be different subnet.
Yes. You basically have four different design options depending on whether you want to have a DR/BDR election or NO DR/BDR election.
See my Quick Facts document for the different options.