Hi Ramil,
Bad Idea!
You should never Join multiple BEx queries in a Crystal Report. You're ought to experience performance issues as the Joins are Not processed on the BW Server. So, essentially, you're bringing in all the data returned by all the queries and joining the tables 'locally'.
You have two options:
1) Create a MultiProvider in BW or
2) Create a Multi-source Universe (If you have BI 4.0) and use CR for Enterprise
-Abhilash