D, 351351 345 bytes
-6 bytes thanx to KevinCruijssen
string f(string a,string b){import std.algorithm;string x,y,z;size_t i,j,k;foreach(c;levenshteinDistanceAndPath(a,b)[1]){final switch(c)with(EditOp){case none:x~=a[i++];y~=b[j++];z~=" ";break;case substitute:x~=a[i++];y~=b[j++];z~="M";break;case insert:x~=" ";y~=b[j++];z~="A";break;case remove:x~=a[i++];y~=" ";z~="R";break;";z~="R";}}return x~"\n"~y~"\n"~z;}