Alias cannot be used, you must setup Virtual IP and Virtual hostname as it specified in the following chapter of the Install Guide. The reason you need to use virtual names and not alias is because the actual IP address gets embedded in the queues, so even if the hostname alias is the same for the 3 servers, the IP will be different and the queues embedded with one IP will not work on a server with another IP. This is the same reason why you cannot change on the fly the IP of a machine running shareplex.
From SharePlex®For RAC Installation and Demonstration Guide
Host names and IP addresses are part of the SharePlex for RAC queue structures and cannot be changed after replication commences. A logical host name, referred to in this documentation as the global cluster package name, must be established and then mapped to the floating IP address before configuration activation. To map the global cluster package name to the floating IP address, create an alias in the /etc/hosts file on all nodes or in a name server (DNS, NIS, NISPLUS). The alias cannot contain non-alphanumeric characters, such as underscores ( _ ) or dots (.). Example /etc/hosts file using the alias hacluster
To verify the resolution of the machine name, issue the UNIX ping command for both the machine alias and the IP addresses. If ping does not resolve the machine name, contact your UNIX systems administrator.
18.104.22.168.6 localsys #permanent IP address
22.214.171.124.7 hacluster #floating IP address