This sentence can be considered humorous or ironic due to its underlying play on words and the unexpected twist it brings towards the end. It leverages colloquial language, usually related to games or war scenarios, and places it in a different context - specifically, a sexual one. In gaming or warfare, "friendly fire" refers to harm inflicted by a teammate or fellow soldier. Here, the phrase is humorously recontextualized to refer to a potentially awkward, embarrassing, or unexpected happening during a three-way sexual encounter. Such a left-field outcome, wrapped up in gaming lingo, can illicit a chuckle due to the surprise element and frankness.
Also, the funny part can derive from the discrepancy between the seriousness of the term "friendly fire", which historically refers to a tragic incident in the military world, and its contextual use to describe a situation that, while potentially uncomfortable, is decidedly less grave. This type of black humor or gallows humor is often used to combat or play down serious situations, even if it proves controversial. The stark contrast of these two things happening in the same sentence produces irony.
Furthermore, it may be considered funny or ironic because typically, in a three-way or any sexual act, the participants are supposed to be working in harmony to provide pleasure to one another. So, when one participant 'gets hit by friendly fire', it is unexpected, thereby creating comedic value through surprise. An expectedly pleasurable act, within an unexpected situation, suggests irony.
In addition, it is a bit of an naughty humor. The sentence doesn't explicitly state anything inappropriate but the implications are clear to those who understand the euphemism. This kind of tongue-in-cheek humor is very commonly used to make people laugh.
Moreover, this situation could be considered comedic through the application of Schadenfreude - that is, taking pleasure from someone else's misfortune. Many people might find it funny to imagine someone else in an awkward situation, resulting in 'friendly fire' during a three-way. It's a sort of an embarrassment humor, wherein the discomfort of someone else causes amusement.
Lastly, the irony may lies in the fact that the individual involved in a three-way - an act that is already considered by many to be a risky venture due to its potential for jealousy, discomfort or breached boundaries - experiences an even less anticipated and typically undesirable event. It becomes a comical surprise in an already risky scenario. This sort of pile-on effect is a common comedy device where the joke builds on previous unexpected turn, resulting in a cascade of laughter. When you re having a 3 way and get hit by friendly fire