Yep.

Clang says:

  zsh% clang -o sequencepoints sequencepoints.c
  sequencepoints.c:7:18: warning: multiple unsequenced modifications to 'i' [-Wunsequenced]
    int r = 1 * a[i++] + 2 * a[i++] + 3 * a[i++];
                   ^            ~~
  1 warning generated.

And prints:

  zsh% ./sequencepoints
  140