how to join /out join a table for datawindow?

In a datawindow, data will coming from 3 tables:

tab1

tab2

tab3.

I try to construct sql with pb 11.5 datawindow graphic mode like:

1. tab1 left join tab2

2. tab2 join tab3

then I can save the datawindow. but when I run the app, I got following error:

Number 11054 Select Error: The Table 'tab2' is an inner member of an outer-join clause. This is not allowed if the table aslo participates in a regular join clause

Not sure why. How to fix it?