Unrelated Greek word? Platonic here refers to the nature of Arthur and Lancelot's relationship. It's a platonic relationship, rather than romantic or sexual one.
Technically the triangle just refers to 'relationships' and not 'romantic relationships' so two male friends and one female love interest works as a triangle but one woman with two male strangers doesn't with because that's just a V.
That is the standard of the love triangle. The two love rivals have to be friends with, or at least know, each other or else there is no tension between them for their love of the 2nd party. Or else every person talking to multiple people on dating apps is in a love n-gon. And love should never intersext with n-gons.