The 3rd grabbing tentacle should unlock as soon as you fuck her anally or vaginally. Oral is not going to work (I disabled that intentionally, because having a tentacle in her mouth disables all the dialog, and I didn't want people to miss out on it).
It was a bit different in the first build I submitted, where it was indeed a bit more obscure. Are you sure you downloaded the new 1.1.1 version?