The particular examples that you cited are relatively recent - though you could argue that recent examples are the ones that would impact teenagers the most.
I grew up in the 80's/90's, and Beverly Hills 90210, Melrose Place, and Sweet Valley High books are pretty comparable examples in that time era. If you aren't familiar with them, they had plenty of love triangle drama. So, if there were an impact on the way we viewed relationships, it would most certainly not be as recent as Twilight.

As you said, the love triangle is very old within literature, so I assume that's not the part you wanted explained.