If you are using CCR - Exchange 2007 then you can have maximum 2 nodes in cluster since it is active-passive file share quorum witnes model.
"Exchange 2007 RTM and Exchange 2007 SP1 support a maximum of two nodes that have the Mailbox server role installed (one active and one passive) in a CCR environment." -
http://technet.microsoft.com/en-us/library/bb124721.aspxIf you are using SCC - Exchange 2007 then you can have more than 2 nodes.