Let

b = the number of boy in the class

g = the number of girls in the class

There are 3 more boys than girls:

b = g + 3

There are 15 students in all:

b + g = 15

We have the following system of equations:

b = g + 3

b + g = 15

........

click here to see the system of equations solved for b and g

........

b = 9 boys

g = 6 girls

There are 6 girls in Preston's class.