They’re all essentially the same person at different points in their long, long lifespan.

Rama-Tut is the youngest, the guy who escaped the boredom of the sterile 25th Century world in which he was born by using Dr. Doom’s time machine to journey into the past, where he used his future technology to set himself up as a Pharaoh in ancient Egypt, before the Fantastic Four messed that up.

Kang is the mature version, an Alexander the Great of time and space who has carved out an empire for himself across timelines and dimensions, and who has battled the Avengers and other folks repeatedly.

Immortus is an older version, one that diverged from Kang at a certain point and became disinterested in conquest, preferring instead to act as a custodian of the timestream.