My vague recollection of reading Delta is that it indeed fits the broad form of the question. A girl named Elizabeth lives next-door to some aliens from Arcturus, who are a species with three genders. I do not recall any details of meeting on the beach, or of car crashes though (I'm not saying they weren't present, just that I don't remember if they were there or not).
[Note: I have now reread the story (see below) and I can confirm that the OP's description is essentially completely correct.]
There is a brief summary here which corroborates these points.
Elizabeth’s simple life as an orphan in her own aunt’s orphanage is
broken by her innocent and desperate attempt to make friends. The
people she befriends, however, are two Arcturians who do not share a
bonhomous relationship with mankind. Her aunt kicks her out yet the
two Arcturians—Imonea, whom she implies is a female, and the handsome
Irveille, whom she implies is a male—take her under their metaphorical
wing. She senses something odd in the triangle until the truth hurts
someone.
... she starts to live under the man’s house and soon discovers that
he has a partner. She accepts their generosity and companionship while
totally ignorant of their alien social customs… then soon finds
herself emotionally involved when she blunders into sensitive social
territory. Still lacking identity yet feeling obliged to their
generosity, she commits herself to their ways even though the layers
of understands are deep and troublesome; regardless, she makes a
decision to find herself through this awkward relationship, be it for
better for for worse.
If you read Delta in English it was almost certainly in Travelling Towards Epsilon, an anthology of French science fiction. This is what the cover looks like - does it jog your memory?
The story is available for free loan at the Internet Library. After reading through it I can confirm that Elizabeth first meets the male-looking Arcturian, Irveille, on a beach. In fact Irveille is what is called a neutral, the other Arcturian sexes being "girl-girl", and "Amazon".
The most classic case was that of a neutral meeting an amazon:
together, they search for the indispensable girl-girl to suit both of
them. As long as they haven’t found her, they constitute a bachelor
‘couple'. More unusual are bachelor couples formed by an amazon and a
girl- girl; even more uncommon are girl-girl and neutral couples.
The other alien, Imonea, is an Amazon, and Elizabeth joins the relationship (she discovers gradually) in the role of a girl-girl.
Imonea crashes her car in despair when Elizabeth clumsily revealed that she loved Irveille, but not her, and goes into a coma. In response
Irveille commits suicide by shooting himself in the head.
Eventually Imonea comes out of the coma, and to give her something to live for, Elizabeth and another Arcturian, Avia, form a triad with her.