设四年级种植树木x 棵.(x -13) + x + (x +22) = 126 ==> 3x + 9 = 126 ==> 3x = 117 ==> x = 39 x - 13= 39 - 13 = 26 (棵) x +22 = 39 + 22 = 61 (棵) 答：三年级植26棵,四年级植39棵,五年级植61棵....