Watch the Lesson 2 online guitar lesson by David Hamburger from Fingerstyle Blues Handbook 1
When you're picking three strings at a time with your fingers, check to make sure all three strings are ringing out at about the same volume. It's easy for the middle string of the three - in this case, the second string, which you're picking with your middle finger - to get lost in the shuffle. Try picking just the second and first string, then just the second and third string. Then add back in the first string to pick all three, and listen for the sound of the second string in the mix.