I really doubt that Johnson is much above 2% or Stein even half of one per cent.
However, both of them could actually take more votes from Clinton than Trump. The Libertarian vote would be the more problematic of the two, but if they can be portrayed as mainly interested in marijuana, that would tend to attract some Sanders type voters away from Clinton. The Green party obviously cuts into the college vote that would normally go to the Democrats, perhaps if they were portrayed as more likely to deliver on climate change than the Democrats, this would not likely affect the Trump vote but could cut into the Clinton totals.
Personally, I sense that this is Trump’s election to win or lose as he will appear stronger than the previous two alternatives, and up against a less able campaigner than the incumbent (although basically the same politics).
I’m not sure he can get to Reagan landslide territory but maybe a comfortable win as per Bush vs Kerry 2004.
And I think Gingrich is the best VP choice. Voters are going to be reassured by having somebody with a proven track record in national politics in Trump’s inner circle. Christie adds nothing and Cruz is probably not interested, and I don’t see him expanding the vote in the right direction necessarily. The Indiana governor did a good job and wouldn’t be a bad choice, but I think Gingrich probably adds the most.
I think Gingrich would be a better chief of staff. He would be the puppet master there.