MS Exchange 2003 cluster

Hi to all,
I've a problem with ABR 11.5 and 3 cluster nodes with 2 exchange 2003 instance.
On the AMS, under "Microsoft Exchange servers", only one instance of MS Exchange is detected.
The exchange agent and the ms patch is installed on all 3 physical nodes.

Cluster topology
Node A-B-C (physical machine)
Exchange instance Alpha-Beta

Node A with Alpha
Node B with Beta
Node C without exchange instance

AMS under "Microsoft Exchange servers" detect only Alpha

I've tried to remove the AMS registration and register again one machine for a time, but Beta instance never detected from AMS.
Can ABR 11.5 supports two or more exchange 2003 instance?
If my configuration is supported, how can i solve it?
Thank you