The big 3 have had the motivational advantage of chasing records (Federer had Sampras record to chase and then had to be wary of Nadal at his heels), Nadal was chasing Fed, Nole was chasing Nadal. And all the big 3 had “down” periods where they weren’t winning much but perhaps motivated by the other two, they kept going on.
So it’s plausible to think that some of the prior ATGs might have won more if they had something/someone to chase.
The ones that come to mind for me are:
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Borg - Retired at 26 in 1983. He was winning slams in 81 and barely played in 82/83. He had equaled Rod Lavers 11 slams which was the open era record at the time.
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Sampras - Retired after winning the 2002 US OPEN (technically he retired in 2003 but the 2002 us open was his last tournament). Sampras played at a pretty high level in exhibition tournaments several years later. Plausible he could win another couple of slams between 2003 and 2005?