Poulidor ... finished the Tour as runner-up three times, but also in third place five times.
Worth noting that 'The Eternal Second' doesn't actually have the most second-place finishes - I can think of at least one rider who has more: Jan Ullrich. Although he did actually win the Tour on one occasion. And there's also the small matter of the historical status of Ullrich's results to take into account.
Also, on a hunch, I just looked up Joop Zoetemelk's results and it turns out he beats Ullrich, finishing second six times (to Merckx twice, to Hinault three times). But he also won it outright once. Can't think of anyone who might have finished second on more occasions.
Poulidor's real distinction is finishing second several times without actually winning it (nor ever wearing the yellow jersey).
Just to get vaguely back on topic, current form suggests Quintana is probably on his way to his third second place in as many attempts, which would be quite an achievement!