I can understand the confusion as we usually only refer to anal sex and vaginal intercourse as penetration. The word penetration is not as commonly associated with fellatio, despite that it involves penetrating the woman's (or man's) oral cavity with a penis.
Fellatio is the act of penetrating the mouth with a penis. Also note that cunnilingus is also considered penetration as the tongue penetrates the woman's vulva. So oral sex on either gender falls under sexual penetration. For reference see:
Also, Wikipedia defines triple penetration so:
"Triple penetration usually involves the penetration of the anus, vagina, and mouth simultaneously"
Most definitions you will find on the internet of triple penetration is of the simultaneous penetration of a woman's vagina, anus, and oral cavity, although other combinations are also possible.
So the use of the term "triple penetration" in the Ai Uehara movie is appropriate. It would likely be marketed as such if it were an American pornographic film as well.