I don't understand why you're coming at this from a US vs EU angle. I've worked in both as well, have had a company in the US, have had a company in Canada as well as several in Europe.
Agreements tend to be legally binding on both sides of the Atlantic, if my experience is any guide then the US is much stronger on contracts and agreements than Europe, but apparently we we're not going to agree on that.
What matters is that we're talking about a very specific case, not just your experience or general matters. And that in that specific case which the linked article talks about there was more than just smoke about there being 3, not 2 co-founders to snapchat and that one of them got unfairly cut out of the deal.
What the US norm is in such cases is not relevant, if you agree on a three way split then that is what you should do. If someone contributes to your start-up in a non-material but essential way then you can decide to reward them or not, but once you've made a commitment towards sharing in the proceeds you can't later backpedal on that.
And there is nothing European about that. Case in point: this lawsuit is going down in the US.