The cost of three packets of pumpkin seeds is 3 x $2.50 = $<<3*2.5=7.50>>7.50.
The cost of four packets of tomato seeds is 4 x $1.50 = $<<4*1.5=6>>6.
And, the cost of five packets of chili pepper seeds is 5 x $0.90 = $4.50.                                                                  Therefore, Harry will spend $7.50 + $6 + $4.50 = $<<7.5+6+4.5=18>>18.
#### 18