while I pretty much agree with deleuran.
a few little points.
1.three finger style was first recorded and used by by Charlie Poole[Who by no stertch of the imagination was a bluegrass player].however you are absiolutely right that Bluegrass banjoists invariably use three fingerstyle,the only one who seems comfortable in more than one style, that I can think of is Ralph Stanley
2,old time fiddlers often use a system of bowing[pair and two seperate for four quavers]This is untypical for bluegrass.