Technically a three letter domain can't be trademarked. Three letters acronyms can be trademarked, but generally not the letters themselves, but just a logo for them. Even then, it's probably only for a specific type of use in a particular industry or product category.
Generally the US trademark office does not consider a domain TLD ending as part of a TM, just as it doesn't consider "inc.", "LLC" or other ending disignators. However, a few names with the TLD were approved back before they made that decision.