Hi,
1. How can i join 3 Datasources to get 4 fields for my Report above?
2. Do we need to create 3 Cubes for 3 DataSources and then create a Infoprovider?
for both 1&2, if 3 data sources are header, item and detailed , then you map all three to one cube and create a report making use of all 4 fileds.
if no single field is common in these 3 data sources, then you have to map (transformation) to separate cubes/targets and have to revise your requirement.
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?
if these data sources are standard (ex: LO data sources) , then you can check the tables (from source : LBWE ) of each data source, you will find any one table in common ..
share your requirement here, to get better response.