OK, this will be just as quick, and perhaps more detailed than you want.
AT&T OWNED the Bell System (Southwestern Bell, Pacific Bell, etc.) prior to 1984.
Prior to 1884 (more or less) you could not purchase a PHONE of PHONE SYSTEM from AT&T, you could only "LEASE" it from the Bell Company.
The Government "BROKE UP" the Bell System in 1984, and part of the deal was that the Bell Companys could not lease phones and phone systems out to end user customers.
Suddenly, AT&T was in the PHONE & System Selling (& Leasing at times) Business.
Now they needed a "NEW" product for end users.
(Their old products wouldn't be good to sell 'cause they were designed to last 40 or 50 years, LITTERALLY!)
Now to get to the point!
AT&T was the ORIGINAL manufacture of the MERLIN. Started in 1983-84 or so. It was only a 2 X 6, (Lines Telephones) eventually becomming a 30 X 70. (ATL Phones)
Later, AT&T came up with the MERLIN 2, bigger, but not much in the way of stuff.
Then AT&T Developed the LEGEND, looked like MERLIN 2, but had the ability to support MLX Phones and ATL phones, and many more features.
Later, AT&T "SPUN OFF" the Phone and Networking stuff, into a company called LUCENT.
Then LUCENT spun off the Phone stuff to a company called AVAYA.
That's why you have 3 different ones.
However, there are some MINOR differences in these phones due to "VINTAGES". I will not attempt to go into these issues, but let's just say the "DP" sets are a little better, usually.
Also, some older "D" sets are just fine too.