Which is why they moved the fight to an enclosed area, where he doesn't have much maneuverability.
Weapons, most of which would have flat out murdered Cap, but that wasn't his goal. And he tried to tie Cap up. It didn't hold.
They pretty much didn't give him any time to react. And two super soldiers teaming up, are more than challenging enough, especially since he was holding back, because Cap was in the vicinity. And even with the 2 pummeling him, they still barely did any damage. Heck, Ironman basically won the fight. Bucky lost an arm, Cap was barely standing. And you have to remember, he was still holding back, because he didn't want Cap dead. He only really lost because he wasn't paying attention.
Or maybe just pay attention and actually watch carefully how they set up the entire confrontation.
Like I said above, blind rage and tactical oversight. When people go red, they get dumb. But on a level playing field, Cap and/or Bucky cannot "beat" Iron Man.
'Sides, he's got enough tricks in that armor to take them out without killing them.