The direct answer to your question is that you have Patients and Laboratory indexed on Patient_id. You set a relationship from Centers to Patients on Patient_id and set another relationship from Patients to Laboratory also on Patient_id. Assuming each of those relationships is one-to-many, don't forget to set skip to the child tables from the parent tables. You can verify that you have it set correctly by browsing the three tables and moving the record pointer only in Centers.
My guess is you could save yourself a lot of trouble by using SELECT - SQL to create a cursor with all of the information you need and report directly from the cursor.
HTH

>I need to develop a report that lists the centers and all the patients that belong to that center along with their laboratory results. The report should pull the data from the following tables:
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>Patients
>Centers
>Laboratory
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>All contains a linked field: Patient_id
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>How can I set relation to all three tables so that once finds all the patients in one center, it'll just move on to the next...
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>TIA
>
>Jeff T.