I'd love to purchase all three volumes in a way that supports the group responsible for the free online version, but when its only $20 more expensive to get a really nice hardcover edition than it is for a set of .PDFs, that's a really tough sell.
If you gave a discount for purchasing all 3, for around $80, I'd be surprised if you didn't increase your sales volume enough to make more money than you would at a higher price.
I'm confused at the marketing where someone else paying for the download bandwidth means it costs quite a substantial sum of money only slightly less than shipping dead trees around, but a continuously updated live website means its free. I would expect for financial reasons the prices of the products would flip.
Expenses are much higher when I read the website than when I read a pdf.
Lets say there was an android app that basically downloaded and cached the entire website and periodically fetched updates / proofread changes. Basically a git client with a crude wrapper, assuming you're using git for source control (or Hg or subversion or whatever). Would I pay $10 for that app? Sure. I think you'd get lots of sales.
If you gave a discount for purchasing all 3, for around $80, I'd be surprised if you didn't increase your sales volume enough to make more money than you would at a higher price.