There are currently 3 * 6 = <<3*6=18>>18 students sitting in the classroom.
There were 3 * 3 = <<3*3=9>>9 students who went to the canteen.
Sally has therefore noticed 3 + 9 = <<3+9=12>>12 students missing from the class.
Elliott said that 2 groups of students have joined the class, so there are another 2 * 4 = <<2*4=8>>8 students who are not in the classroom.
Lucas said that there are 3 + 3 + 3 = <<3+3+3=9>>9 foreign exchange students who are not in the classroom.
Therefore, there should be 18 + 12 + 8 + 9 = <<18+12+8+9=47>>47 students.
#### 47