Generally speaking the sensors you are referring within Open Hardware Monitor are differing sensor temperatures and can have varying meanings on a per system basis. The temperatures "1, 2 and 3" you're referring to don't correspond with cpu cores 1, 2 and 3 for your system in open hardware monitor. With that being said, in the image you provided if you look at the cpu temperature section located under AMD Phenom II X3, your maximum core temperature is is recorded as 47.5 degrees thus, ruling out the cpu core as "Temperature #3" which is stated as a maximum value of 89.0 degrees.
There is a possibility that the temperature recorded (89.0 degrees) could be anything from system, cpu socket, northbridge or other on board sensors etc. Regardless, I do still think you should investigate a bit more into exactly what those temperatures mean specifically in regards to your system for future monitoring purposes utilizing Open Hardware Monitor.
For a bit more insight as to how to interpret the information in Open Hardware Monitor the links below should give you the tools necessary to determine what they mean. You will need to utilize your motherboard user guide (To determine available on board sensors) along with another monitoring program (HWInfo) to determine exactly which sensor that is by comparing side by side and then rename it in Open Hardware Monitor for future use.
Gigabyte GA-880GM-D2H User Manual
HardwareInfo (HWInfo) Another fantastic freeware alternative to Open Hardware Monitor which presents the information in a more detailed and clearly explained manner.
I recommend using HWInfo when you require detailed system information (More resources intensive) and Open Hardware Monitor as your daily (less resource intensive) monitoring tool for basic system monitoring. Like I said above, utilize them both to determine which sensor temperature #3 is by performing a side by side comparison.
I hope this helps bring you a bit of clarity to your issue and have a nice day.