There is an EXCELLENT, EXCELLENT (can you tell I like it? ) application called MenuMeters, which posts a variety of real time updated meters in your Menu bar. You can monitor CPU, memory, disk, internet, etc. I couldn't live without it, and because is all shows up on your menu bar, it is always there, always visible, and takes up no additional screen "real estate".
I am not a G3 expert, but most modern Macs have a variety of thermal sensors built right into the system. My G5 has a small army of them - CPU, GPU, connecting areas... it goes on and on. Can any current G3 owners comment on whether the G3-class machines were also so equipped?
So, since these are usually always present, apps like MenuMeters can simply read and display the results.