Halo 3 graphics wise stood the test of time during the 360 era. Was still one of the better looking games years after release, at least IMO.Apart from Halo 4 and maybe 5, Halo was never a top tier looker. Go read any review of Halo 3 for example and you'll see most comment on the graphics being a let down.
Infinite is a cross gen game so of course it looks like one.
Definitely true that Halo 3 wasn't a massive upgrade over H2 though.