Watch the Fretting With Thumb online guitar lesson by Gareth Pearson from Travis, Atkins & Reed Guidebook
Hopefully by now you're able to fret two strings at once. So now, try adding your thumb to fret more of these additional Travis type chord shapes. This is where it can get really difficult, so go easy on yourself, give yourself time, and be patient. And keep practicing, playing these shapes all over the neck!