Nobody ever addressed this matter so I will: the scene in Civil War when Tony and Nat are discussing possibile allies to stop Steve and the Rogues just before Tony goes back to New York to recruit Peter.
Nat says they’ll need help, and Tony asks her if she knows where Bruce is and... I don’t know, but thinking about it sounds a lot like OT3. Civil War is a Ironwidow shippers event only and I don’t accept criticism on this (well, Ironwidow and Stucky to be super honest), but that scene specifically?? It’s like the ultimate conclusion of: Tony and Nat’s flirting in Iron Man 2, Tony and Bruce’s ongoing relationship after Avengers and Bruce and Nat’s escapade in Age of Ultron! That’s what wrap it all! It’s basically them having an healthy triangle open relationship!
In which Stephen would fit perfectly and you can’t tell me otherwise!