Can you list the champions that have this:
around 800hp+ when at 33% HP around mid game
around 800hp+ when at 33% HP around mid game

And more.
midgame could mean anything from like level 7 to level 16, so that's a bit too broad of a question..
when building 6 warmogs
Literally any tank that builds for it. Bonus points for the tanks that get free health (Singed, Vlad, Cho) or that have abilities that scale with health (Mundo, Voli, Malph). Strengh of ages also really helps and
is OP for that goal.
May we ask why?
Pretty much any Cinderhulk jungler.
Cinderhulk + Strength of Ages + Scaling Health Seals will typically bring you fairly close to 3k HP by the mid-game.
Anything that can have a working Warmogs passive at mid game.