Watch the Two - One - One Forms online guitar lesson by Brad Carlton from Guitar Lab: The Golden Arp
This lesson covers the two-one-one form of the add 9 arpeggio in root position. This form is very useful for rhythm guitar parts as the 9, 3 and 5 can be sustained because they each fall on a separate string with this form. This layout also facilitates legato single note playing because you can combine a hammer-on or pull-off on the lowest string, and then use sweep picking for the remaining strings. Another great option is to implement hybrid picking. I'll explain the reasoning behind certain fingerings which will improve your intonation and increase the flexibility in your fretting hand. This 2-1-1 form is useful with all four three-string groups. It's easy to visualize as you are simply taking the root position major triad forms and raising the root one whole step for the add 9.