I havent used a 3 groove yet I think I herd where S1 is using some 3 grooves. I would call Dan Lilja and talk to hem or email Dan http://www.riflebarrels.com/ . My 6.5/284 is a Krieger 1 in 8.5 twist 28".
In two 30 cal cartridges, 7.62 NATO, and the 30 Wolf, The Lilja three grooves tested superior in 3 areas, Accuracy, Velocity, and Throat Life. The Krieger 4 grooves were close in accuracy, and velocity, not close in throat life. The Hart and Schnieder and Obermier were also close in accuracy. The Black Stars finished last in every catagory but throat life, and were not even close in the accuracy department.
My .22-250 has a 3 groove Pac-Nor 1:8 twist that is proving to be a good choice in accuracy and throat life. My 6.5-284 also has one identical to my .22-250 except for bore size, obviously. Its too early to tell with this one, but I'm pretty happy with it so far.