OT 3 finger style is basically pre-Scruggs, e.g. Snuffy Jenkins and a few others. I mentioned in a thread on 2 finger picking Art Rosenbaum's excellent book Old Time Mountain Banjo. I don't have it in front of me, but IIRC it has a short section on OT 3 finger picking. A major difference between OT 3 finger and Scruggs style is the use of slides, hammer-ons, POs, and other ornamentations in Scruggs style. You don't see them nearly as much in OT. Listen to Snuffy Jenkins, and you'll hear a pretty significant difference. He sounded, to my ear anyway, like a two finger picker who just added his middle finger occasionally. In other words, he sounded more old-timey, not bluegrass.
I think there are several OT threefinger styles. First, the original threefinger style that was popular from about 1870 is often called classic banjo today. It is simply finger style guitar applied on the banjo. Watch the videos on http://classic-banjo.ning.com for example, to get an idea of this style. Also try to check Vess Ossman's cylinder recordings from around 1900: http://www.archive.org/details/VessLOssman. Secondly I think Charlie Poole, mentioned above, has an own style, based on some of the classic banjo techniques. Charlie Poole made his recordings in the late 1920's and early 30's (I don't rememberwhen he made the last records). Third, I think Frank Jenkins has a style of his own, combining arpeggio chords and melody notes. I think Kirk McGee has a similar style (try to listen to his version of Snowdrop where he mix threefinger with two finger up-picking). Fourth, also Uncle Dave Macon used a lot three-finger picking in his early recordings (1920's and 30's). His style is similar to Frank Jenkins and Kirk McGee and he mixed three-finger with several other styles. Fifth, Dock Boggs should also be mentioned. He plays a simple style based on T I M I rolls combined with twofinger picking (similar to the socalled Seeger's basic strum) and simple thumb - pinch pattern where the thumb notes often follows the melody. Sixth, there are other styles that I have forgotten for the moment, or that I havn't hear about.
It can really be discussed what the difference is between OT thfee-finger and Scruggs style. I think most of the rolls are present in the OT music, but Scruggs developed the music. In the OT music banjo is often either the sole instrument without backup (listen to Frank Jenkins and Uncle Dave Macon , or it is used as only accompaniement in a band situation (Charlie Poole). Scruggs developed the banjos role to be both a solo instrument in the band and to be a vamping instrument. I also think that Scruggs was the first, or one of the first, to use the forward roll more elaborated throughout a sole. Scruggs also developed a more syncopated styles than usually was used by the OT players, and it was to a large extent the dorward roll that induced the syncopations.
All I'd add is that when the majority of people who have heard of OT 3 finger think of OT 3F, they think of Snuffy Jenkins' style of picking. It's easy to hear the influences on scruggs, minus the fancy syncopated rolls, idiomatic licks, and ornamentations. Also, OT 3F doesn't have the nonstop roll that scruggs has, but has interruptions of single 8th notes, as opposed to the continuous 16th note rolls.
Also, Wade Ward has a beautiful and complimentary style of 3F backup-to-fiddle style. I'm still figuring out the nuances, but it sounds amazing and further solidifies my belief that good 3F banjo is the best backup to fiddle (besides a good OT guitarist) (think Earl Scruggs and Paul Warren).