How hard is it to get a good to great grades in undergraduate school? I am currently studying undergraduate school in Europe and am toying with idea of applying to a U.S. collage for graduate school. 

So I‘ve read a bit online about GPA requirements and there it says that one should have a score of at least 3.0. My current GPA would be way below that and I‘m not a crazy outlier at my school. All of my friends have grades similar to mine.
At the same time I know a few friends in America who all never have grades worse than a B.

So do I just coincidentally know two absolutely non-overlapping groups on either side of the Atlantic is it easier to get good grades in America?
It is hard to answer this question as the educational systems in the US and the EU are very different at the undergraduate level. A bachelor's degree, say in mathematics, in the US includes many other courses such as writing, history, philosophy, etc. Only about half of the program is mathematics. In the EU the degree is normally much more focused on maths. But for any given course in maths, I'd guess that the level of difficulty is about the same, though this varies widely from place to place. 

So, a GPA (overall) in the US means something quite different from one in EU. The GPA in field is a more comparable number. 

But, with more focus in the EU, some courses that would be taken in graduate school in the US show up earlier in the EU. 

You have to work hard on both sides of the pond to achieve success. Don't expect lower standards in either place.