Assuming that X trees are planted in the fourth level, the third level is (X- 18) and the fifth level is (X- 18+30).

X+(X- 18)+(X- 18+30)= 180

x+x- 18+x- 18+30 = 180

3x-6= 180

3x= 186

X=62 trees

Then plant 62 trees in grade four, 44 trees in grade three and 74 trees in grade five.