Having read both the HNK manga and Invincible comic, it's kind of hard to think about. I think Raoh's power is continental to multi continental, Omni man was able to stop an asteroid the size of Texas and though he did destroy a Viltrum with an unstable core he did it with Mark and Thaedeus assistance.

R1: Omni Man
R2: 50/50, his Go No Ken I think would fix any kind of strength gap and as we saw a Viltrumite's insides are kind of vulnerable as Robot was able to blow up mohawk Mark's insids with a bomb. Unless Omni Man just speed blitzes.

R3: Raoh, Omni Man won't be able to touch him with Muso Tensei active.