The ART120 has an ART special inlay at the 12th fret only, while both the ART300 and ART320 have diamond inlays at frets 1, 3, 5, 7, 9, 12 (ART special) ,15 ,17, 19, and 21. The ART320 has a flamed maple top, the ART300 has a caiman finish, and the ART120 has a bound maple top. From what I read, the ART120 is only available in the Asian and European areas.
Here's links for the ART120 (Ibanez Wiki) and the ART320 (Ibanez Official Website):
http://ibanez.wikia.com/wiki/ART120
http://www.ibanez.com/ElectricGuitars/model-ART320