when you say cluster, you mean jboss cluster, right? Otherwise, you can point the 3 standalone servers to the same database schema, each one will be part of pega cluster (not to be confused with jboss cluster). pega does not use any of the jboss cluster features.
since pega is not using jboss cluster, it is better not to use it until further notice (you can certainly file an enhancement request for our future releases). There may be some incompatibility issue since we have not tested the configuration.