By Kayla Kibbe As we’ve previously established, planning a threesome with your partner is often easier said than done. First there’s a bunch of mental and emotional preparation you have to do to make sure you’re both ready to invite a third person into your bedroom — and then of course you actually have to find that third person, which is usually the hardest part. It’s for these reasons — and probably many others — that I assume Fire Island star Joel Kim Booster prefers to skip all that labor and simply be the “guest star” in other people’s threesomes. The actor shared his ménages à trois pref…