Connors, Borg, McEnroe, Wilander, Edberg and Becker combined>>>Federer, Nadal and Murray and I say this as a huge Djokovic fan.
Now hold on one second here. Lets not lump that all into one group and think Lendl was facing all of these guys at once.
Lendl caught the very end of Borg. His first two seasons in the top 10 were Borg's last in his career. He denied Lendl of 1 slam and Lendl was not a consistent top tier performer yet.
When Lendl came up in 82-84, becoming a top tier player and started hitting his prime, his main competition was Connors/Wilander/McEnroe. Connors took two slams off Lendl in this time and Wilander took 1. McEnroe dominated 84 shutting everyone out besides Lendl who won his first slam off McEnroe.
By 85 Connors and McEnroe both declined and neither would ever win another slam ever again and were shells of their former selves. Tennis had just gotten so big during the late-70s and early-80s that Mac and Connors were content to stick around making QFs of every slam and getting bounced for the publicity and recognition and then just fighting to win lower tournaments (250/500). At this point they basically devolved into a modern Tsonga/Berdych/Ferrer level players.
From 85 on saw the rise of Becker and Edberg to contend with Wilander and Lendl. Pat Cash was merely a modern Del Potro at best and Yannick Noah a Stan Wawrinka.
So at any given time Lendl had 3 chief rivals
80-81 was Borg/Connors/McEnroe (Borg retires end of 81)
82-84 was Connors/McEnroe/Wilander (Mac/Connors never legitimate top guys starting in 85)
85-forward was Edberg/Becker/Wilander
80-81 was monstrous with Borg/Connors/McEnroe, but Lendl was akin to 06-07 Nole at this time and only really lost out on 1 legitimate title due to the era (prime Fed at USO 07 vs prime Borg at FO 81).
I would say the rotating window of big 4s Lendl was part of is equally as tough as the consistent Fed/Murray/Nadal Nole's had to face for his career. (So far at least, it will be until Fed totally falls off the planet and Dimitrov/Raonic/Wawrinka don't prove to be suitable replacements).