As with any reply, your mileage may vary.
Would it be normal/acceptable for a couple to go into a gay club in order to pick up either a man or a woman to 'play' with for the night or weekend?
In part that will depend on what the local GLBT community is like and the club as well. Some will be more or less accepting of that behavior. I've been to several clubs where trolling was ok. And some where the regulars didn't care for it. Not every GLBT group/community is the same.
No one really likes to be used and discarded as a sex toy. That's kinda what it comes down to.
Some of the behavior was dictated by how much they knew the couple. If it was a regular duo then sometimes they found a partner for a night of wild debuachery.
I've rarely seen strangers hook up for a good bit o'fun. It happens regardless of what some say or think.
How would a kinky couple go about finding a third person??
I ask for my novel, I promise.
Do they have my email address yet? Just kidding.
Back home the local porn stores have about.............6 rows of locally published swing magazines, running from 10$ up to 50$. They list people who have a desire to get freaky with one another. We even have a private swingers club. Generally by invitation only. Single guys need not apply. Couples are ALWAYS welcome.
That or just go to a bar, find some drunks guys and eventually you'll find one who'll do it with the couple.