Or… the child’s behaviour was much more disruptive than the OP thinks/is describing. We’ve no way of knowing for sure.
Personally, I am inclined to think that the parent who doesn’t ensure that their 3 year old can’t swing on their chair repeatedly is probably not aware of a lot of other elements of their child’s behaviour that are irritating and disturbing other diners (e.g. they were shouting/screeching - “but they’re only 3, all 3 year olds do that”, etc.)
So many parents now seem to think that because something is developmentally normal it shouldn’t be corrected or prevented. Then they wonder why they have an older child who behaves poorly at school or home. Just telling a 3 year old to stop swinging on their chair won’t work because they’re mostly not developmentally able to act on that instruction - it’s always better to phrase it with what you want them to do, not what you want them not to do, in any case (“Keep all the chair legs on the ground”, even “Sitting nicely, thank you” is better). More importantly, you have to actually stop them (arm on the chair, sit them on your lap, take them away from the table if that doesn’t work). Then they will know that when you say “We need to keep all the chair legs on the ground”, you mean it.
OP, if you can’t control your 3 year old now, you’re potentially going to have big problems in a few years time. You absolutely can control any 3 year old. Ultimately, if he doesn’t do what you expect then you (as gently, calmly and kindly as possible, while explaining what you’re doing and why) pick him up and remove him from the situation. He has to know that you are in control, both for his behaviour and development and for his long-term happiness. If you can’t pick him up (disability/health issues), then it’s harder but you need an equivalent strategy.