Dexter have 9 x 15 = <<9*15=135>>135 basketball cards.
He filled 9 - 3 = <<9-3=6>>6 plastic boxes with football cards.
So, he has 6 x 20 = <<6*20=120>>120 football cards.
Therefore, Dexter has 135 + 120 = <<135+120=255>>255 cards in total.
#### 255