If I have 3 years of back taxes to file and each year has a different address that shows on my T4 Slips, respectively, do I input the most current address for all 3 yrs?

You would put your current address for all years that you are filing for. The CRA will send any mail they need to send you to that address, so if you put a past address, it will go to the wrong place.

If you haven't already updated the CRA with your current address, you need to do so before you can NETFILE. You can update your info through the CRA’s My Account service or by calling them at 1-800-959-8281.