Below is my understanding :
- In weblogic 10.3.6 you have a cluster of three managed servers and you have deployed the application ( reports ) on the cluster.
- The same reports app works fine on a single node WLS.
- There might be a problem with the cluster communication / a network issue.
- Are you using multicast / unicast.
Check the cluster --> monitoring page on WLS console, check if the Managed servers are moving out of the cluster frequently.