Actually, the CPU temp sensor is the best measure of the temp inside the CPU chip, but case temps (more important for northbridge and southbridge chips of the mobo, and for GPU cooling on a graphics card) are better measured with a sensor in the case. For this purpose most mobos use a sensor built into the mobo itself somewhere. In this way they can run two independent temp control loops. One is for the CPU cooler system, and is guided solely by the CPU's internal sensor. The other is for Case Fan or SYS_FAN, and it is guided by the mobo sensor. A few cases have two SYS_FAN outputs, but I'm not clear whether they use separate temp sensors, or share one as their guide.
The unit that uther39 linked to is designed to use the mobo's CPU cooling fan output to control three fans, but as I said they must be 4-pin fans, and the mobo output port must be 4-pin type to provide the PWM signal. Normally, this wiring unit powers the CPU cooler on one of its outputs, and the wires also convey the CPU cooler fan speed signal back to the mobo. The other two outputs are for other fans, and they simply will mimic the CPU fan's actions. They could be case fans, in which case the case cooling is being driven by the CPU's cooling needs, as you suggest. Another way to use this wiring accessory, though, is to connect its leads intended for the mobo's CPU_FAN output port to a 4-pin SYS_FAN port and then run all three of its outputs to case fans. In that situation they all would be guided by the the case cooling controller and the mobo's temp sensor. As you say, you can't use it that way because you don't have a 4-pin SYS_FAN output port on your mobo.
That wiring device is relatively cheap to produce because it does not have to reproduce the PWM signal - it merely lets up to three fans share it in parallel, and that works because each 4-pin fan motor's internal controller chip loads that signal line only minimally. However, to build a circuit that takes one 0-12VDC signal and produces three or four such signals, each capable of supplying a fan motor with up to a couple of amps (for start-up) takes much more in design and components, and probably would cost as much as existing third-party fan controller modules. So it appears nobody has decided to try to crack the market with such a device.
As regards the quality of the posts here, you are right. There are many people who are real experts on particular things, and lots of people with experience in your exact problem almost every time. I like to go where there are people who know more than me so I can learn and get help, so I stick around here, too.
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