"But you can't ship three dragons! That's so weird!"
Polyamory. Polyamory. Polyamory. Polyamory.
That aside, I view this ship pretty much the same way I see Sunnyspeaker; there's nothing wrong with it, but there's others I prefer. Given that they were a love triangle at one point, I feel that having Sunny, Starflight and Fatespeaker all in a relationship would be a bit awkward, from a realistic character development perspective. Sunny and Fatespeaker would get a bit jealous over Starflight, and Starflight may find he loves one more than the other. It'd most likely end in tears.
In an AU, maybe it could work. They could definitely help each other. Sunny would be like the mother of the relationship, responsible and doting. Starflight would be the nervous one and would probably get incredibly blushy and shy around them. Fatespeaker would be the most enthusiastic one. They'd probably get up to all sorts of cute activities, like reading to Starflight or helping out at Jade Mountain or chatting together during the RainWings' suntime.
Good chemistry, but the context makes it unlikely that they'd succeed together.
5/10.