Not many do. Rog;lic usually tapers off in the third week as did Froome quite often until he stopped attacking so much in the mountains. Quintana at his best had good third weeks and Pogacar just stays good throughout but in the last Tour he did look tired in the last few days but still good enough to withstand attacks. This race is looking more like the 2020 version . I expect the overall to be close unless something decisive happens in the TT but even then the gaps probably won't be too big.
"usually" meaning got sick once and was once running on fumes in November after being in red hot form from June?
2018 Tour - climbing his best in the 3rd week, but suffered in the ITT from a rock in his elbow
2019 Giro - got sick, which is well known, was already suffering as early as stage 4 and then equally bad in week 2 and 3
2019 Vuelta - No problems uphill in week 3
2020 Tour - best climber in the Alps, got widely flamed for not burying Pogacar there
2020 Vuelta - Was in red hot form since June, still beat mythical 3rd week Carapaz in the third week
2021 Vuelta - Did you see the Covadonga stage?
What really happens is you see something happen once or twice, then it becomes this self perpetuating myth about a rider and it never really gets questioned.