Kai pointed out that the perceived difference in bitterness may have been due to reduced hot break in the kettle for 3X. The wort for 3X was noticeably clearer running into the kettle, so there may have been less hot break to bind the hop acids.
This is interesting. I wonder if this means a triple decoction is worthwhile for a massive IIPA.
If you're thinking about it increasing the bitterness, I'd have to say there are easier ways.
I'm thinking specifically about IIPA's that are aiming for the 100+ IBU level, where you are close to maxing out on IBU's and you want to cram as much as you can in there. It would be interesting to see how many more IBU's you can squeeze in using a decoction, if any.
I agree for most hoppy beers, it would be a heck of a lot easier to just use a couple extra ounces of your bittering hop (or maybe some Hop Shot) to get the same result. I'm just wondering if we can somehow raise the upper limit on IBU's.
Maybe if I feel adventurous some day I may boil then chill a batch of wort to get the hot break and cold break to fall out, then rack off the trub and re-boil for 90 minutes with like 300+ IBU's worth of Hop Shot just to see what happens.