Watch the Exercises for Practicing 3rds online guitar lesson by Allen Hinds from Melodic Improv
Learning 3rds is going to help you see the fretboard. So many of the books and schools (in my opinion) teach you in little boxes, so students end up playing in little boxes. I try to develop exercises that bridge positions on the guitar. It's important to see the guitar in one big position, without barriers.
In this lesson, we'll be playing our thirds again, focusing on ways to get out of the box. This works for other intervals, too: Try it out with fourths, sixths, etc. or mix them up to see what kind of sounds you can get out of it.