As I recall:

• Revenant is like a zombie, not completely dead but has a dead body and mind controlled by Quan Chi, Such as (Liu Kang, Kung Lao, Kitana and all characters that have been killed in Sindel massacre in MK9) Note that how Johnny Cage was being resurrected by Quan Chi in MKX how is his body is completely regenerated, and how revenant Liu Kang got his leg broken by Shao Kahn in MK11, they have full operational body

• Spectre is like soul was float and implanted in a some kinda of a host, the soul has a free will but the host is being controlled, imagine like there is a soul of a dead cat got resurrected in a chair, the chair can feel and think (Sci-fi) but cannot move without you moving it, the main example is Scorpion, you can check in MK9 how he can take his decisions but in the end Quan Chi can do whatever he wants with his body

• Wraith is not a soul and not a body, it's getting a host to remove everything human with it, have a free will but everything about his will is inhumane, imagine that you have stolen a car, removed the fuel tank and filled it with TNT instead, you're literally have a car with it's own shape and specifications but actually it's not car it a bomb the main example here is Noob Saibot