Arrange these masses from the heaviest to the lightest.

1. Convert $3\text{ kg } 108\text{ g}$ to kg in decimal form

$3\text{ kg } 108\text{ g} = 3 + \frac{108}{1000}\text{ kg} = 3.108\text{ kg}$

2. Convert $3\frac{1}{8}\text{ kg}$ to decimal form

$3\frac{1}{8}\text{ kg} = 3 + \frac{1}{8}\text{ kg} = 3 + \frac{125}{1000}\text{ kg} = 3.125\text{ kg}$

3. Compare the three decimal values and arrange from heaviest to lightest

The three masses in kg are $3.180$, $3.108$, and $3.125$.
Comparing them: $3.180 > 3.125 > 3.108$.
So the order is $3.18\text{ kg}$, $3\frac{1}{8}\text{ kg}$, $3\text{ kg } 108\text{ g}$.
This matches Option (2).