Jim,
If you purchased The DCS Companion 3rd Edition in March of 2017, you purchased it from MTH via the Kibo/Shopatron website.
In order to access the download and get another copy, you'll need to login to Kibo using the same E-mail address and the password that you used when you purchased the 3rd edition. This password will open the new copy that you download.
Regardless, unless you changed the password since purchasing the 2nd edition, the password for the 3rd edition should be the same as for the 2nd edition. If it isn't the same but the E-mail address that you used for Shopatron in the past is the same, you can get a new password by going to this link. There, you'll be E-mailed a temporary password that you can use to log in to Kibo, setup a new permanent password, and download another copy of the 3rd edition.
This new copy will open with your new Kibo password.
If you no longer have access to the E-mail address that you used to purchase the 3rd edition, you'll have to call Kibo and work things out with them directly. The phone number to call is (877) 412-7467.
Let me know how you make out via the E-mail address in my profile.