I was wondering if someone could write me up a cog that is client/server that hides 3dos when they are not in view (like if there are 3dos behind other 3dos)...

On the colosseum, even when indoors (in the corridors) the FPS is down to about 13... I think this might be able to be fixed using a cog.

It has to be client/server though, because it is a MP level

Client/Server? Why? What do you want to synchronize?
Also I think it's impossible to write a 3do occlusion detection cog. The only values cog knows about the models of the things are the center and the radius of the containing sphere.
What could be done, though, is a process that hides 3dos which are outside the player's viewing angle. Question is if this is necessary at all or already being done by the engine.
Another possibility would be to implement something similar to the thing creating system GBK once wrote. You could then cut your map up into sectors and use them to define visible subsets of your architecture. All other objects would then be hidden before the engine even gets a chance to render them. This requires quite an effort from the leveldesigner, though.

Yeah, if you cut your master sector into a pie, it'll only render 3do's in visible wedges (so u'll be cutting the number of 3do's rendered by more than half from some angles). No cogs would be needed, cuz JK automatically does not render 3do's in sectors that are not visible. If your 3do's are wedge-shaped, you shouldn't even have to re-export your models.

Yeah, but a modified version of GBK's thing creation system should be able to hide 3dos behind other 3dos which are INSIDE the viewing frustrum.

How so?

THAT'S what I really need.. for 3dos to HIDE other 3dos... because if you are inside the arena, you can't see all the arches because of the bleachers.. but the framtrate is just as bad.. as if you COULD see all the arches ont eh OTHER side of the bleachers

Well, I'm sure there are cases when a 3do is completely behind another 3do, but still in the looking direction of the player. Why not take notes of all these situations, then "zone out" the regions in JED using sector arhitecture. These sectors could then be used as triggers for a cog to hide/unhide the 3dos behind other 3dos...
If you combine this wilth LOD work, that features screenshot-textures on openings in the lowest level of detail (you know, taking a screenshot of what can be seen through the opening, then changing the opening to a solid wall and applying the screenshot as a texture on this wall), you shouldn't notice anything.
I'll repeat that this could be quite some work, though, involving a lot of tweaking.