Going on your thinking there's no need for anything bigger than a 3 cylinder v 4 cylinder.
Oh dear, oh dear: you'll have to explain how you can extrapolate from "3-cylinder engines are no less reliable than 4-cylinders" to claiming it's the equivalent of saying "there's no need for anything else".
Any engineering design decision is based on a multiplicity of requirements and a multiplicity of potential designs to meeting them: there's usually no one perfect design (if there was, all engines would converge onto it). Engine reliability is simply one of those requirements. 3-cylinder designs are better at meeting some requirements (their weight reduction influencing fuel economy and reduced cost of manufacture springs to mind) and worse at others (relatively poor noise, vibration and harshness or NVH). Fine if NVH is important to you and your gut tells you not to buy, it's your choice. But that choice is simply part of your buying decision in purchasing. Others will be less concerned with the drawbacks and more concerned with the benefits of reduced fuel consumption, lower cost and so on. Because 3-cylinder designs tend to be better at requirements more important in smaller cars and worse at those more important in larger cars you will tend to find them in smaller cars and so larger cars will tend not to use them. So no, I'm not saying there's no need for anything else. For the same reason, larger engines again tend to have more than 4 cylinders, which have a second order reciprocating balance issue requiring (costly) balancer shafts to correct as the size of the pistons gets large enough to make it apparent.
The OP asked a question about the reliability of 3-cylinder designs. I've pointed out that there's no inherent reliability issue with them, with reliability far more governed by individual design, usage a particular car is subjected to and whether or not it's been maintained to schedule. By the way, I should have said maintain "at least to schedule": I'd agree more frequent oil changes are a good investment if longevity is important to you, though with modern oils not as much as it used to be.
I've also recommended looking to evidence to establish the reliability of a particular model (or track record of manufacturer if the design is relatively new). On that, you should take a look at an independent source of information such as Warranty Direct's index, based on warranty claims in the UK. See [broken link removed] where a model with a 3-cylinder option just happens to be #2 on the list of the most reliable cars.
Now ranged against a point of view that says look to the evidence, we have "perfect sense" and now "gut feel". That's fine if it's just an input to a buying decision; gut feel is what most people use in deciding to purchase a car (me included by the way: it's not a completely rational decision by a long shot). But please don't use notions of "common sense" or "gut feel" to claim somehow that of course 3-cylinder engines are less reliable unless you can point to some evidence to back it up.