Is it necessary to book interisland flights in advance? If so, how far in advance? Have several weeks to visit islands and have not decided on third island. (Flying into Honolulu and plan to go to Big Island, torn between Kauai and Maui).

flying to Maui or Kauai you should have no problem finding a seat as they fly just about every hour. Getting to the Big Island you should make advance reservations as the flights are limited and sell out (except first flight and last flight)

Also, there are limited flights that fly direct between the Big Island, Maui and Kauai. Chances are you may have to fly back to Honolulu and transfer to another flight to get to the other island. You should book your flights as soon as you know your schedule. If you happen to be flying during a 3-day weekend or other holidays, flights will be booked up. Sometimes there are statewide school tournaments or state conventions and flights can get booked up. Avoid weekends if possible.

island air has good deals on iterisland flights - 39 one way. I used my united miles (5000 for RT) for inter island flights with aloha or Island air.

If you want a certain flight/time, I would book in advance. We arrived early in Honolulu and wanted to catch an earlier flight to Kauai than the one we were scheduled, and all the flights were fully booked.