How can i configure GRE Tunnel on router A so that it will be terminated on Routers B and C which are running HSRP. This is required such Router A is always connected to the Active HSRP Router A or B whichever one that is Active HSRP for failover.
HSRP ------------------- Router A
I am afraid you won't be able to use the HSRP address as the GRE tunnel source/destination address. The router would have to use one of the addresses configured on an interface as the GRE source but HSRP is just a virtual address and therefore it isn't a viable option. You may have to look for other ways to provide a primary/failover solution.