I don't understand... Is it a program that isn't allowing Diablo 3 to be used to its full extent in the GPU thing? I mean, is there something you know that I didn't do? I closed skype, steam, firefox, flash player, etc. I closed everything I besides Diablo 3 and the system & internet proccesses... I don't know what to do anymore seriously

Is there a way to allow more "threads" to diablo 3? Maybe it has a limited number of threads to 50 or something? Is there a way to make it so that Diablo 3 can use all the CPU it wants? Or force it? I'm out of idea

No... closing your other programs is not going to free up GPU (graphical processing unit, which is mostly used for rendering games like Diablo 3), rather pretty much just CPU (main processor). None of what you're describing is going to have any effect on your GPU. Don't even
look at your other running programs, they're not causing this, they're only using your CPU, which in your screenshot is hardly in use so those programs should be perfectly fine. Diablo 3 is using your GPU. You are running too many Diablo 3 instances at too high of a quality in too high of a resolution, for your GPU to keep up.
According to what you have shown me, you have plenty of CPU for everything you're running, but
you're overworking your video card. You can't tell Diablo 3 to use more CPU, it's not even TRYING to use more CPU because it's GPU limited not CPU limited. It's trying to render more, but does not have the GPU power to do it.
I have to point you back at my previous post, with the following suggestions...
Okay so with that in mind, first double check each game window's video settings to see that you didn't set one to low but leave the other at high quality. Turn off anti-aliasing and set low textures and particles, etc.
Next I might consider dropping your FPS limits in ISBoxer to 25 to reduce some of the strain. In-game limiting can be disabled/unchecked.Here is another way I explained this in another recent post...
If you want to improve framerate, see HOWTO: Tweak your framerate. See "Identifying framerate bottlenecks" and check your GPU, CPU, and RAM loads at the time you're having trouble. If you're losing framerate, then most likely one of these is hitting 100%, and that is your limiting factor, aka bottleneck. If your GPU is at 100%, then to smooth your framerate you need to decrease your typical GPU load so that the spikes don't hit 100%. You can keep ALL of your framerates at 30 instead of letting the main window at up your GPU with 60, for example. By giving up some of your FPS on your own, you are reducing the amount you're going to hit the limit.