Iron Man can't transform into other armors, that's just some BS from the cartoon so they could sell action figures.
Now just to clear up some inconsistancies. The War Machine armor is technically an Iron Man armor, infact it was originally called the Iron Man Variable Response Threat Battle Suit and Jim Rhodes went by the name of Iron Man at first while wearing it before Tony Stark came out of a coma and allowed Rhodes to keep the suit. War Machine and Iron Man have fought each other toe to toe but since Tony Stark built the suit he knows its weaknesses and Iron Man armor has advanced much farther than that since now and War Machine is gone, Jim Rhodes retired from being a super hero and last the War Machine armor was scene it was found by a criminal which put up a good fight against the more advanced Iron Man armor of the time, proving the War Machine armor is a good design but nothing the suit's creator can't handle.
As for the fight, Colossus isn't exactly in the same catagory as Iron Man and Steel. They're both technological heroes where as Colossus is a mutant. As it was said Colossus would have the dissability of not being able to fly and Iron Man's strength does exceed Colossus' (Iron Man has stood up to both Thor and the Hulk) and Steel is just a cruddy DC knock off of Iron Man and doesn't have the armaments and experience to take Iron Man down, plus Iron Man's armor is always improving as technology improves. They don't call him "The Invincible Iron Man" for nothing.
Shellhead is the clear winner in this one.
The Ultimate universe was mentioned, and it could go differently if you take the Ultimate stats where IM is severely wussed down in my opinion. Going with the original universe characters Iron Man who was always one of the strongest Avengers wouldn't have a lot of trouble.