3a)

$\overrightarrow{u}\cdot \overrightarrow{v}=0+0+0=0$
$\overrightarrow{u}\perp \overrightarrow{v}$

b)

$\overrightarrow{u}\cdot \overrightarrow{v}=2+1-3=0$
$\overrightarrow{u}\perp \overrightarrow{v}$

c)

$\overrightarrow{u}\cdot \overrightarrow{v}=2-4+15=-6+15=9$
$\overrightarrow{u}\cancel{\perp }\overrightarrow{v}$