The software that comes with the motherboard is quite decent nowadays, so there's that, or SpeedFan. Try the motherboard software first, SpeedFan is a headache to get setup, but does work well, eventually.
Idle temps are responsive to cpu output, ambient temps, case temps from airflow and your cooler.
So a temp of 45°C isn't out of the ordinary for some ppl. You can check the airflow status by pulling the side off the pc and blowing air in directly by a desk fan or such, if temps at idle drop considerably then airflow in the case is an issue. Very small change or even temps going up means airflow is great.
The 3000 series has had a few bugs related to bios versions as regarding temps vs voltages etc, so that might be worth looking into as well.
But 45°C isn't terrible on a stock cooler.