3. Teen Tony
To combat the evil Iron Man, the Avengers time-travelled themselves and recruited a teenage version of Tony Stark (who hadn't yet been controlled by Kang) to help them. Because, obviously, the best counter to one of the smartest men on the planet is a version of him with far less skills and experience.
As the above paragraph may have tipped you off, it didn't go well. Teen Tony stole a suit of Iron Man armour to fight his older self and was badly beaten. However, the sight of his younger self caused Tony to regain control and he sacrificed himself to defeat Kang.
After that, Teen Tony just kind of stuck around in the present and took the original Iron Man's place. Thanks to the injuries he received fighting the evil Iron Man he had to have a reactor fitted to his heart, just like his older counterpart. He even gained control of Stark International because, in a world where anyone can come back from the dead at the drop of a hat, ownership laws just fly by the seat of their pants.
Teen Tony would later die along with the rest of the Avengers to stop the villain Onslaught. They were all brought back though by Franklin Richards (reality warping son of Mr Fantastic) and Iron Man was restored to adulthood.
None of the other Avengers ever mentioned any of this again.