The short answer is that it depends on your cold tolerance. Some may find a 3 mm just fine, others are still cold in a 7 mm.
Personally, I dove Kona last August in a 3mm full and did just fine, except the second dive of the day I noticed I got just a little chilled towards the end. My dive computer put the bottom time at 65 minutes so we were down just over an hour. My daughter took her 7 mm and did fine with no complaints, even though it was a little small and we didn't zip the back up all the way.
If it doesn't tip the scales for excess weight take the 3 mm and if you need you can rent a 5 mm. Or, take along a neoprene vest to layer under. If you have the 5 mm I'd recommend taking it but wouldn't recommend buying one just for Kona unless you think you can use it on other dive trips later.