So I have a budget of $140 what is a good midrange fx cpu for a asrock 970 extreme 3 mobo
With your budget, FX-8320E seems to be a good choice.
95w, under load, the CPU temperature is relatively low, which is a good thing.
For everyday computing, the performance is satisfying, as well as gaming.
Of course you can go for an 125w+ CPU like FX-8350 for better performance,
but it also brings more heat and higher power consumption.
Agree with the advice above, the FX8320E is a solid choice.
Asrock like many mobo makers lists all the CPU models supported and the necessary BIOS update required to run a specific model CPU. The BIOS may need to be updated before you can run a newer CPU model. Check the Asrock website for Supported CPUs for your specific model mobo and any mobo Revision, (Rev).