Ken Boyer got his career off to a great start after missing out on his first couple of seasons due to millitary service. He was a very good 3B for the St Louis Cardinals, but played just 10 seasons before becoming a role player for the Mets and White Sox after that. He hit .287, 282, 1141. In 2034 games, he had 2143 hits, 1104 RS, 318 2B and 68 3B. All numbers could have been closer to what Santo finished with had Boyer not missed time due to the millitary service or stuck around for a couple of more seasons in his prime. While Santo was borderline (and IMO slightly above the border), Boyer has to be belong the border in this one. He deserves having his number retired and the respect his name gets in St Louis but he's just not a Hall of Fame player.
Another interesting 3B was Graig Nettles. Nettles had a very good career, mostly with the New York Yankees. The difference is Nettles played in more games than Santo and Boyer. Nettles finished with 2700 games, 2225 hits, 1193 RS, 328 2B, 28 3B to go along with his 390 HR and 1314 RBIs. He hit more HRs than Santo and scored barely more runs in over 400 more GP. The main reason Nettles will not be a Hall of Famer is his batting average, just .248 for his career. Dave Kingman played some 3B in his career, hit 442 HR but hit only .236. He is not a Hall of Famer. Darrell Evans, who was predominantly a 3B, had more walks than strikeouts, hit 414 HR, but is not in the HOF because he hit .248 for his career, the same as Nettles.
I understand why these discussions come up. I can see a Cardinals fan watching Boyer play when he was with the Cardinals or a Yankees fan who watched Nettles play for the Yankees think these guys belong in the Hall. The were great players, but they do fall short of the Hall of Fame.