Lisa,

            I don't even know how you got to 3 entries before having
problems.  I just spent about 15 minutes trying to get to a new item
in the edit dialog for a List under Thunderbird and it was not easy. 
I finally happened upon the following, see if it works for you.

            After you've entered the latest address to the list, hit
the left mouse button or NVDA's equivalent, NumPad Divide [desktop
keyboard].  This creates a blank next entry in the list directly
beneath the one you've just entered, then you can hit TAB to have
focus thrown to that blank entry.

            When you've entered the last address you need to enter,
hitting TAB without doing the left mouse click will throw focus to the
OK button to save the list.  If you accidentally didn't create the
next list entry, this is where you'll be thrown when you hit TAB.  You
can SHIFT+TAB to get back to the last entry you made, then hit END to
get to the end of that e-mail address you entered, then follow the
usual procedure of left clicking (or equivalent) to create the next
blank entry, to which you can then TAB.
--

Brian *-*Windows 10 Home, 64-Bit, Version 1809, Build 17763

*/A great deal of intelligence can be invested in ignorance when the
need for illusion is deep./*

          ~ Saul Bellow, /To Jerusalem and Back/