I got awkwardly confused with this problem at first

Yeah but the centre of mass of a chair with a person sitting on is definitely not in the middle, so the pressure being to the side of the idealistic position is more likely to cause the chair to tip due to the increased distance between the legs which provides more torque. So its less stable

You're assuming that all of the legs are equal lengths and that the floor is perfectly flat and level. Yes, in a perfect world, more legs = greater stability. But when accounting for the imperfections of the real world it's easier to balance on 3 points (at least when the object in question is static), because it takes exactly 3 points to define a plane. If we're talking about, say, a moving vehicle like a tricycle, then yes it is absolutely less stable than a 4 wheeled vehicle because there are other vectors to consider besides purely gravitational (at least in a Newtonian sense... ok n/m I'm overthinking this).