Earlier quoted context omitted.
Perfectly common in large parts of Europe. I also have a 3 months notice period, which is pretty much the standard here in Sweden for tech workers or other highly-qualified occupations. For very senior positions, 6 months isn't unheard of. It goes both ways of course, my employer is also bound by the 3 month period in case they want to fire me.
How does getting a new job work? You tell the new company that you have to give 3 months notice? That's some long term planning on the part of the hiring company.
There are contractor agencies filling the niche of providing employees on short notice (and usually for a short period), but generally having people begin 3 months later is just something everyone expects and factors into their processes.
The average length of time people spend at a single job is also longer than in the US.