There are 2 classes x 5 days = <<2*5=10>>10 classes on weekdays.
There are 4 classes x 2 days = <<4*2=8>>8 classes on weekends.
There are a total of 10 + 8 = <<10+8=18>>18 classes per week.
So each week, 18 x 5 = <<18*5=90>>90 people can take classes.
Thus, 90 x 3 = <<90*3=270>>270 people can take classes in 3 weeks.
#### 270