Hi,
Please find below answers.
1. How can i join 3 Datasources to get 4 fields for my Report above?
A. 1) You can create Cube and load data from all three DSO to Single Cube and create report on Cube.
2) You can also create Multi provider to join the data on these three DSO create report on MP.
2. Do we need to create 3 Cubes for 3 DataSources and then create a Infoprovider?
A. No need to create three cube refer answer of querstion no 1.
3. How to check if 3 Datasources be linked to each other? i mean which T-code can we use to check the Relationship among Datasources?
A. You need to identify Key fields as per your requirement and create Cube or MP. once you create you must check data in it whether is it correct or some data is mismatch(No T-Code is for checking Joins or relationship )
Thanks,
Kashif Ali Khan