Hello,
I am afraid I will have to disappoint you.
Every HSRP speaker needs its own unique address in the subnet, plus one shared IP address that is used by all HSRP speakers as the virtual IP address of the standby group.
However, if all 14 IP addresses in your /28 subnet are exhausted already, you cannot add another HSRP speaker to the subnet because there is no unique address available for it even if it shares the IP address of the HSRP group, because it still needs its own unique IP in addition.
The same goes for EIGRP.
Without getting at least one more free IP address in the subnet (which means either readdressing or moving some hosts out to another networks), I am afraid there's no way you can have a first hop redundancy protocol there. HSRP requires N+1 unique IP addresses where N is the number of routers (every router has its own IP address, and the entire HSRP group has another IP address). VRRP requires N unique addresses because in VRRP, the virtual IP address can match the address of one of the routers. But you still need to have the unique address for the added 3750.
Best regards,
Peter
