Figure is equivalent to the one shown below.
It is a Wheatstone's bridge in which
$P=\frac{6 \times 3}{6+3}=2\, \Omega$
$Q=\frac{8}{2}=4\, \Omega$
$R=\frac{4}{2}=2\, \Omega$
and $S=\frac{20 \times 5}{20+5}=4\, \Omega$
We find that $\frac{P}{Q}=\frac{2}{4}=\frac{R}{S}$
i.e. the bridge is balanced. Effective resistance between $p$ and $q$
$R_{A B} =\frac{(P+Q)(R+S)}{P+Q+R+S}=\frac{(2+4)(2+4)}{2+4+2+4}$
$=\frac{6 \times 6}{12}=3\, \Omega$
