Method 1 (direct method): "There must be male teachers and female teachers among these three teachers", which are divided into two categories: one female teacher and two female teachers. The selection method of one female teacher is C 4 2 ×C 3 1 = 18, and the other two female teachers are C4 1× C32. Therefore, "there must be male teachers and female teachers among the three teachers", and there are 18+ 12=30 different options, so the answer is: 30. Method 2 (indirect method): The different selection method of selecting three teachers from four male teachers and three female teachers is C7 3 = 35, and the selection method of all three teachers is C4 3 = 4. There are 35-4-1ways to choose different teachers, so the answer is: 30.