why 3 sensors on exhaust

I am guessing you have a V6 in the Ranger. There are two pre-cat O2 sensors and one after the cat O2 sensor. They help the computer set the fuel/air ratio.

Thanx,do they all go bad? 3 @ $50.00 is stiff for disabled vet that has little funds

Of course they go bad but not necessarily all at the same time. If the CEL is on have the codes read which may tell you which one(s) are bad.

They may be good so first check all the possible reasons for one to set a code if one has been set and you think you need new O2's. I will post a url copy & paste it to a new browser window. It will give information on what to check for, Like you said all going bad at same time is most often something causing them to set code so check all possible causes before replacing.

https://www.engine-light-help.com/oxygen-sensor-codes.html