it’s the polyamorous flag! polyamory means you may be in relationships with more than 2 people
a lot of people consider it cheating, but it’s actually not because 1: if person A is dating both person B and C, B and C consented and were ok with that, unlike how they may feel if it was cheating. and 2: person B and C are also dating eachother
a 3 person polycule is usually called a “throuple” :)