I was diving in Maui early June. The water temp was 78. I had a 3mm full, and it was perfect. I had a Lavacore top+bottom but didn't use them, but I wish I had them on instead of just a swimsuit while snorkeling in the shallows off the beach.
The dive operator's rental gear was 3mm shorties. Some were OK with that, but I noticed that a lot of people were cold (cumulative effect of multiple dives, breezy surface intervals, the water temperature and personal biology). I think shorties are just cheaper and easier for tourists to manage.
I'm no expert, but my observation is that if it's warm enough for a shortie, it's cool enough for a full 3mm, but the reverse isn't so true. A 3mm is a widely useful tool, provides better protection from stings and scrapes, and you can always let some cool water into a full, if needed. I haven't yet used my Lavacore gear enough to have a firm opinion, but my sense is that it's in the same category as a shortie - whenever it's warm enough for a Lavacore, it's cool enough for a 3mm full. I think the Lavacore (or Sharkskin or Frogskin brands...) has more utility as a layer under a 3mm. And then, if you already have it, you can use it for protection or a little bit of warmth instead of just a swimsuit, but I don't buy the marketing language that it's almost as good as a full 3mm.
The 3mm full is the Swiss army knife of warm-ish water diving. My suggestion is to get one of those first.