The r3's are softer then the m2's and m3's but last longer.. its a new compound,, heres alittle info from there site

"M3 is the much softer compound of the two and is used when more traction is desired. The M2 will give you better tread life but less traction. The newer R3 material is just as soft as the M3 but will give you an increased tread life".