The original shooting title of the movie was FACADE, which is ultimately much more apropos to the filmmakers' intentions, because all the characters are engaged in a performance of "Two Faces Have I": Todd Pearson is the teacher but he really knows nothing, matronly Olive Millikan wants to enjoy sex as much as the students she keeps in line, student Sharon McClure just plays at being a bad girl and really wants to be loved, and Professor Di Fermi...well, he's keeping a bunch of secrets. TRIANGLE may have been the sexier sounding title to get the pruriently curious in the door, but it's false advertising: only until the last third of the movie is there even a whiff of some sort of relationship triangle, and it's not even isosceles.
TRIANGLE is not an effective drama. As much as it wants to be depicting "edgy" sexual behavior, it's all too tame -- even a wine party sequence likely inspired by John Frankenheimer's SECONDS doesn't deliver any kind of turn-on. Unfortunately, it's not even good for camp value, because all the performances are reasonably well-acted and quite restrained; there's nothing over the top to ridicule. You can laugh at Paul Richards' perpetual sullenness or Tiffany's bravado, or the portrayals of the "big issues" of the time, but those titters dissipate into general boredom over the course of viewing.
What is interesting to watch in this movie though is Tiffany Bolling. She's compelling, pretty, and definitely makes an impression as a good actress. It wouldn't be later, until her unofficial drive-in trilogy (THE CANDY SNATCHERS, BONNIE'S KIDS, THE CENTERFOLD GIRLS), where she would get to fully display her range and become a cult movie icon.