This site may give you some answers:
In general, ARC models improve with each revision - at least that's ARC's position.
Until recently, I don't think there was much debate that the Ref preamp of choice was either the Ref 2 original (MkI) or the Ref 2 MkII. There are a couple camps among ARC fans; one camp is fully in synch with ARC and they will tell you that the Ref 2 MkII is an improvement over the Ref 2 MkI. Others, who in my opinion are also thoughtful customers with good ears, will say that the Ref 2 Mk I is a superior sounding preamp. The thinking is that the the use of the all tube 6922 design in the Mk I provides more "musical" sonics than the hyrid JFET approach used by the Ref 2 MkII. This same debate carries over to many of ARC's preamps and amps. You can read people's opinions on the all tube (6922) designs vs. the hybrid JFET designs on Agon and audioasylum.com forums.
Recently, ARC introduce a new Ref 3 preamp. The early feedback is very positive.
Ref 3s will probably cost close to the list price (about $10k USD new); it will probably be difficult for a while to find them on the used market. Ref 2 Mk2s go for $5-6k or so used depending on condition. Ref 2 MkIs probably go for about $4500 or so used.
In general, while everyone has their opinions on the last nuance of sonic attributes, ARC equipment is extremely well made, provides excellent sound, and is backed by some of the friendliest and most responsive service in the industry. Whichever ARC model you select you are likely to be happy.
One other item - all the Ref preamps are line stage preamps; if you use a turntable you will need a phono preamp. ARC's PH5 is an outstanding phono preamp - a notable improvement over the very good PH3SE; in fact, some people would say that the PH5 is sonically competitive with the Ref Phono despite being less than half the price. If the Ref 3 is as big an improvement over the Ref 2 MkII as the PH5 is over the PH3SE, the Ref 3 must be a tremendous preamp.