I've enter from both Nong Khai and Mukdahan around 10 time each since 2010, on either tourist visa or visa exempt entry (quick bounce), and it was never an issue. Most friendly border to enter. Nothing is ever asked, no proof of, nothing. I never ever showed any of the "3 proof of", this is usually reserved to air entry. What
@Brandon ***********
is describing by land is probably only true for certain nationalities that get quizzed often, but for G7 countries like me (Canadian), and other rich nations, you don't get ask anything by land unless they want to fuck you over in Poipet, for example.