Hi,
don't panic,
If Speedfan reads it as 79 degrees constant from cold boot ,then it's probably safe to assume that particular temp sensor isn't connected to anything.
Ok let me explain that for you,
Temp 2 is your CPU temp, if your cpu was running at 70 degrees we wouldnt be talking to you right now from that pc.
Speed fan 3 is dodgy with some motherboards and gives bogus readings such as temp 3 & temp which arent really connected to any device, its just picking up signals from something it thinks is a temp sensor,
Most likely it's a temp sensor that's not utilized, but the sensor chip has a circuit for it. Just go into configure and uncheck the box next to that temp and it won't show up and scare you anymore,
Just in case, though, you might want to verify the temps by checking the temperature monitor in the bios.
The drop in case temp with the side panel off, means that it could do with front cooling fan/s pulling more air into the case, you have the 2 grills spare in the front,
Regards