I was kind of thinking of writing this as a sort of stepping-stone in Amythest's sexual awakening.
Rose is imperfect and occasionally somewhat selfish; she thinks of this as her pleasure, then Greg's, then Amythest's, though she does keep their enjoyment in mind throughout. Ideally, she just wants everyone to have a good time and for Greg to introduce another gem to the wonders of the newly-named Murdercock.
Greg sees it primarily as indulging his girlfriend, but also legitimately exploring feelings of sexual attraction he and Amythest have felt for one another for quite some time. He wants above all to ensure she feels safe, relaxed, and free of expectations. Secondary on his mind is satisfying Rose's deep-seated fetish
Amythest is just stoked at the prospect of sleeping with Greg and Rose. She's very interested in fusion/sex in general at this point in her development, in spite of her relative inexperience. She trusts Rose and Greg and feels completely safe exploring her sexuality with them. This experience is what leads her to be far more comfortable with sex and casual relationships themselves.
Hope I didn't flub this explination, this is a lot more detail than I usually go into for simple fapfics lol