The probable problem is a ground loop. This is is difference in the ground potential in the house's power wiring. You are most likely using a different power outlet for the third sub than the rest of the system.
There are a few ways to eliminate the hum. First is to run an AC extension cord from the sub to the same AC outlet as the rest of the system. You could also lift the ground to the sub by using a grounding plug adapter.
Be carful about using this as the ground is there for safety.
The third way is to use a signal ground isolation transformer. There are many of these at various prices.Blue Jeans Cable
sells a couple located at the bottom of their subwoofer cable page. There are many listed on Amazon