In-Universe, Captain America, Iron Man, Thor, although i would put Fantastic Four as the icons of mainstream superheroics, in comparison to vigilantes(Spider-Man,DD) or outcasts(X-Men).
In real world, pre-MCU, it was Spider-Man, Hulk, Wolverine, with Thing falling to fourth position(before Wolverine, Thing was certainly the third most famous marvel hero). Now, i feel it's Iron Man, Spider-Man and Wolverine.