YES

   1: +(+(x1,x2),x3) -> +(x1,+(x2,x3))
   2: +(x1,+(x2,x3)) -> +(+(x1,x2),x3)

@Jouannaud and Kirchner's criterion
--- R
   1: +(+(x1,x2),x3) -> +(x1,+(x2,x3))
   2: +(x1,+(x2,x3)) -> +(+(x1,x2),x3)

--- S
   1: +(+(x1,x2),x3) -> +(x1,+(x2,x3))
   2: +(x1,+(x2,x3)) -> +(+(x1,x2),x3)