I want to specify model ID, study ID and case number in a script and export the results in csv.

When using GetDataSet in the DataManager class will give the same result in all cases.

You can specify case numbers by using GetDataSet in the Study class.

Enter GetDataSet(dataset name,case number).

An example in a Python script is as follows.
# -*- coding: utf-8 -*-

app = designer.GetApplication()

#10 case torque for the second study of the first model.

ref1 = app.GetModel(0).GetStudy(1).GetDataSet(u"torque",10)

#Create the graph specified above.

graph = app.GetDataManager().CreateGraphModel(ref1)

#Export csv file.

graph.WriteTable(u"D:/test.csv")