Watch the Two - One - One Forms online guitar lesson by Brad Carlton from Guitar Lab: The Golden Arp
The final inversion of the 2-1-1 forms of the add 9 arpeggio is 3rd inversion. This form is rather stretched out but well worth the effort. Start by touching the 1, 2 and 3 on adjacent strings. Then visualize the 5 five frets lower on the lowest string. "Seeing" this information is the mental challenge just as stretching is the physical challenge. The beauty of this layout is that the 5 can function as a pickup note, and the 1, 2, 3 cluster can ring together for a great close voicing keyboard sound. As always be acutely aware of your intonation since these big stretches can cause problems.