That was the 4th finals run, not the 3rd. In the first one in 2015 he was substantially outplayed/outproduced in the finals by 20-year old rookie Teuvo Teräväinen. That team btw was largely propelled by the emerging Triplet Line out of Syracuse, New York.
When people say he's been underwhelming in the playoffs (compared to his supposed stature) it's because he's been outproduced in various runs by teammates like Tyler Johnson (2015) and Ondrej Palat (2022).
He's been on several very long SC playoff runs, most often on a team loaded with offensive unicorns, but he still hasn't hit 20 points in a playoff run, which is borderline impressive in itself, especially being stapled to that power play. And this is someone where the scoring/numbers is supposed to be the bread & butter.
What may be an underrated aspect of Stamkos is his leadership ways. Always when I listen to this guy in pressers he comes across very mature and level-headed. A guy like Kucherov on the other hand could never be captain on any successful team IMO, just way too grumpy, immature and emotional.