23.731 g rounded to three significant figures is 23.7 g.
Rule for significant figures:
All non-zero numbers are significant.
In 23.731, there are 5 non-zero numbers. So, there are 5 significant figures. To get number in 3 significant figures round the number at the first decimal place.
Now, the 23.731 g can be expressed in 3 significant figures as 23.7 g.