I tried to use BGP and it totally worked fine. But any multicast traffic is not working. I checked Wireshark and packets are being encapsulated on a GRE packet twice.

Router1:

interface FastEthernet0/0
 ip address 21.1.77.1 255.255.255.0

interface Tunnel10
 ip address 1.0.0.1 255.255.255.0
 no ip redirects
 ip mtu 1400
 ip pim dense-mode
 ip nhrp authentication cisco10
 ip nhrp map 1.0.0.3 21.97.10.1
 ip nhrp map multicast 1.0.0.3
 ip nhrp map 1.0.0.2 203.177.7.1
 ip nhrp map multicast 1.0.0.2
 ip nhrp network-id 10
 ip tcp adjust-mss 1300
 tunnel source FastEthernet0/0
 tunnel mode gre multipoint
 tunnel key 10

Router2:

interface FastEthernet0/0
 ip address 203.177.7.1 255.255.255.0

interface Tunnel10
 ip address 1.0.0.2 255.255.255.0
 no ip redirects
 ip mtu 1400
 ip pim dense-mode
 ip nhrp authentication cisco10
 ip nhrp map 1.0.0.1 21.1.77.1
 ip nhrp map multicast 1.0.0.1
 ip nhrp map 1.0.0.3 21.97.10.1
 ip nhrp map multicast 1.0.0.3
 ip nhrp network-id 10
 ip tcp adjust-mss 1300
 tunnel source FastEthernet0/0
 tunnel mode gre multipoint
 tunnel key 10

Router3:

interface FastEthernet0/0
 ip address 21.97.10.1 255.255.255.0

interface Tunnel10
 ip address 1.0.0.3 255.255.255.0
 no ip redirects
 ip pim dense-mode
 ip nhrp authentication cisco10
 ip nhrp map 1.0.0.1 21.1.77.1
 ip nhrp map multicast 1.0.0.1
 ip nhrp map 1.0.0.2 203.177.7.1
 ip nhrp map multicast 1.0.0.2
 ip nhrp network-id 10
 tunnel source FastEthernet0/0
 tunnel mode gre multipoint
 tunnel key 10