I have a cave with a big lake in it. The entire
cave is only 2 sectors. I wanted an island in
the center but wanted to keep the sector
number at two rather than the 50 sectors
it would be to make the same cave with the
same island out of architecture. I made the
island with a 3do but it is in two sectors.It
doesn't flicker on any of my computers. Do I
need the 3do flicker cog or is it just fine how
it is?

It should be fine. The flicker cog is a workaround for a "feature" of the JK engine, where a 3do is not being rendered, when it's originating sector is out of view. Since you have two large sectors, this should never happen with your island. It was a huge problem with small sectors and large 3dos, before GBK invented this brilliant cog, though.

I have made a level with the EXACT same concept, I had an underground cave with water and islands/stagmites poking up all over the place. To solve the flickering problem, I simply cleaved my 3do sectors (before I exported them) in half at the point where the water line was going to be, then placed them VERY close together (too close to possibly notice in game) with the two 3do's meeting directly at the water's surface. You should consider this if you ever encounter the flicker problem.

Well that's that then. I haven't had any problems at all with
flickering so I won't worry about it. Thanks guys!

To clarify the cause of the issue, it's caused when the player exists in a different sector from the 3do and the 3do's origin goes out of view.

I thought it was when the 3do's origin sector (the sector which contains it's origin) is going out of view...

Generally, usually, almost all the time, when the sector that the thing is assigned to goes out of view, the things in that sector are not rendered (I don't believe I've had a case where this didn't happen)

However, I've had one (maybe more) case when I was in the same sector as an actor that I killed and he was not rendered when his origin went out of view.

So... it's uncertain as of yet exactly how JK decides whether or not to render an object, but it is usually 99% of the time when the thing's assigned sector is not in view.

I stand by my statement.

However, I will say that while it is possible to be in a sector but that sector be out of view, the engine will continue to render all objects (and continue maintaing their template flags) in that sector regardless if the sector is in view or not. You can test this by cutting the default box in half, place a walkable object in the center of sector 0 and place the walkplayer on top of that object facing sector 1. If the above wasn't true, you'd fall as soon as you loaded into the level (because the walkable object wouldn't be present). But obviously, if the things assigned sector isn't in view, the of course it's origin won't be in view either.

I don't think this is possible. The current sector is probably always treated as visible.

Actually I believe it is quite possible -- camera offsets

Camera offsets are possible. Assume from now that the 'current sector' is whatever sector the camera is in. You can always see the current sector. The engine still 'sees' adjoins, even if it doesn't render them. You don't need to see a surface in a sector for it to be visible.
I've noticed the behavior of corpses, too. JK must have two ways of doing it. Since corpses are temporary anyway, it treats them differently.

Captbevil - Which objects are update is not determined by which are in view. And you can't exclude an object from physics calculations just because the player can't see it. Your example has nothing to do with rendering, and the alternative - falling through the 3do - would be a rather nasty bug.