Conversion from Star Connection to Delta Connection:
From the Star Connection
Resistance between terminals 1 and 2 when 3 is open circuited :
⇒ R12 = R1 + R2
Likewise, resistance between terminals 2 and 3 while terminal one is open circuited.
⇒ R23 = R2 + R3
In the same way resistance among terminals, 1 and 3 is
R13 = R1 + R3