Answer
Top to bottom:
$Cyclohexane$
$H_{2}O$
$H_{g}$
Work Step by Step
Cyclohexane is less dense than $H_{2}O$ and $H_{g}$ so it will float to the top.
$H_{g}$ is more dense than $cyclohexane$ and $H_{2}O$ so it will sink to the bottom.
The $H_{2}O$ will be in the middle.
