I've been to the islands six times now - two weeks at a time. Only once did we do three islands and I wouldn't do it again. And that was 12 days. Too much disruption on travel days, with worrying/planning for the next trip, it just impacts everything.
I'd pick two - at most - from Maui, Kauai, and the Big Island. Even 8 days on one island (I'd recommend Maui) isn't too much. I've spent 4 one-week stays on Maui and am just starting to run out of new things.