There is a nice map of all current international overland border crossings in this region in the middle of this page
The rest of that page and the site has lots of useful info. It is not really very difficult if you stick to the three countries you mention. If you want to go to Vietnam you’ll need a pre-arranged visa which is not difficult, it’s only if you want to go to China or Myanmar that it starts to get interesting (i.e. more difficult).