I wouldn't do anything until I used a megger. Then you will probably get some reading (in megohms) from A & C as well. And your B reading may decrease. At that point you can compare the readings, and decide if any action should be taken.
Insulation quality testing can be complicated, as there are other tests that can be done(Insulation Spot Test, Dielectric Absorption Test, Step Voltage Test) which may provide more insight. A manual provided with the instrument will help determine the necessary tests.
I am no expert on insulation testing but I have done it a dozen or more times. Each time I do it, I review the techniques & procedures.
[This message has been edited by Redsy (edited 02-04-2003).]