I am looking at F3 and F3 HP. Is the difference between the two entirely contained to a prism? In other words, simply swapping a regular "eye level" prism for HP prism my regular F3 will become F3 HP?
I googled but couldn't find the definitive answer to this question.
Yes.
My 2nd question is just my curiosity: why are F3 focusing screens so thick and heavy vs FM3a/FE2 thin & simple screens? Visually I can't tell much of a difference, they're both great.
As pointed out, the F3 screen has an acrylic screen and a glass lens, the ones on the FE/FM series are only a piece of acrylic.
The DE-2 finder (non-HP) finder is preferable in most cases: Greater finder magnification and lower profile. I use glases and there are no obvious advantages to me of the added bulk of the HP finder.
Yeah, the HP finder became standard fairly early on - and is also a fine finder. I just wanted to point out that it is a common misconception that the HP finder is somehow better. Few people have used them side by side.
I had the F3 with the non-HP finder, then the one with the HP finder and also the rather rare F3P camera which has a titanium HP finder with hotshoe.
I liked, by far, the non-HP finder. It is smaller, lighter, and has bigger magnification. I need +0.5/1.0 diopter to be able to really nail focus, and it's far easier and convenient to screw an eyepiece diopter. Plus it makes the image even bigger.
However, neither of those finders is better to the quality, brightness and size of the image provided by my Nikon F2SB camera. This reason, among others, made me selll my last F3 camera.