COnsider the data values as follows:
R1 R2 R3
--------------------------------------------------------------------------
A A D D
-------------------------------------------------------------------------
1 1 1 1
. . . .
. . . .
20 20 20 .
1 21 .
. . .
. . .
10 30 40
SO, R join S will give max of 30 tuples and that join with T will give max of 30 tuples only because T contains unique tuples.