Yes. 3 gallons of 80 IBU beer will make 6 gallons of 80x3÷6= 40 IBU beer, diluted.
The only thing you need to watch out for is the maximum IBU's. Theoretically, that's 110, but Russian River says they've never been able to get past ~90 IBU, no matter what they tried. There have been some other studies that have topped off as low as the mid-50s, but that seems unusually low.