Three companies are all neck-and-neck in the race to $2 trillion.Artificial intelligence (AI) is responsible for adding trillions of dollars in value to a handful of companies over the last few years. Nvidia, for example, briefly touched a$5 trillion market cap this year, thanks to its dominant position in the market for graphics processing units (GPUs). Four other companies sit firmly above the $2 trillion threshold as we approach the new year.But three AI stocks currently have similar market caps around ...