How could we retrieve all the values (master data) of status, severity and priority fields of IBM Rational Team Concert(RTC) using Java API?

well, severity and priority are enumerations.. so you can get the list from
(See my sample code at post 3 of https://jazz.net/forum/questions/94776/assertionfailedexception-problem-with-getting-the-values-of-attributes)
                           
                                    List<ILiteral> enumerationLiterals = enumeration
                                            .getEnumerationLiterals();

you have to get the states list from the WorkflowInfo object

here is some code from an advisor plugin
<code>
                // get the worker objects
                IAuditableCommon iac = ((ISaveParameter) data).getSaveOperationParameter().getAuditableCommon();
                WorkflowManager wfm = new WorkflowManager(iac);
               
                // reference the right object type (cast)
                IWorkItem workItem = (IWorkItem) auditable;
                // get the workflow this workitem is in, so we can get the labels of the states
                IWorkflowInfo x =wfm.getWorkflowInfo(workItem, monitor);

                if(Debug) System.out.println("the workitem state is "+ workItem.getState2().getStringIdentifier()+ "=" +x.getStateName(workItem.getState2()) );
                // if this workitem is going into resolved state (we accessed 'proposed new' state above
                if(x.getStateName(workItem.getState2()).equalsIgnoreCase(ResolvedState))
</code>

>i want only one method for retrieve the values

sorry, RTC doesn't not provide a data neutral single method.