Never been in a threesome but I'm lucky enough to have friends to have been involved in such arrangements.
On the words of a very good friend and sexually experienced woman: if the women are not the slightest interested in being with each other, there is a 90% failure chance.
MMF trios are easier to manage. Yes, men are basic amd as long as they can put their thingy inside a hole, everything is fine. Some guys will use the woman in the trio as an excuse to "feel" bisexual territory. Others will see the other as a competitor and up their game. Usually, the woman reaps the bounty.
FFM trios are trickier. If the women are in to some bisexual interaction, it can smooth things over. If not, the guy really has to go the extra mile to be available for both and deliver. And even then, the probability to let one or all parts feeling unsatisfied is very high.
Last but not least: can't say if Buttercup is playing dirty or just being kinky but... damn.
There's a very relevant scene from the show Scrubs, but unfortunately my memory of it is vague and googling what I remember is just returning nurse themed porn.
Yeah, when it's just a case of 3 people who all want to fuck both of the others, it's kinda hard to go wrong.
Though it also works best if at least 2 of the participants are active lovers. Two starfishes would be more like the one person takes turns fucking the others. Three starfishes would probably just end up being a threesome agreed upon that never ends up happening.
At least one person needs to be bold enough to make a move.
Same advice applies for that as for all other things related to relationships and/or sex:
Put yourself into situations where things can happen. (Can't win the lottery if you don't buy a ticket.)
Don't expect anything to actually happen at any given moment. (If you are only there for one thing other people will notice it and that is not attractive at all.)
Be patient and enjoy whatever else is going on meanwhile.