There are 3 * 20 = <<3*20=60>>60 green marbles.
There are 60 * 20/100 = <<60*20/100=12>>12 yellow marbles
In total there are 3 * 60 = <<3*60=180>>180 marbles.
That leaves 180 - 20 - 60 - 12 = <<180-20-60-12=88>>88 marbles of another color in the box.
#### 88