In vicinal-dihalides, the two $?Cl$ atoms are present at adjacent positions. In case of $C _{3} H _{6} Cl _{2}$ only one such structure is possible. Hence, only one vicinal-dihalide is possible for $C _{3} H _{6} Cl _{2}$
$CH_{3} ? \underset{\overset{|}{Cl}} {C}{H}?\underset{\overset{|}{Cl}}{C}{H_{2}}$
1, 2-dichloropropane