I would assume/hope this means you have a senior role and are well paid. This is not uncommon for members of a leadership team etc to ensure reasonable transition times and stability. Also most employers will show flexibility here in my experience. E.g. A couple of jobs back an employee just requested I be on call for a couple months for the new person should they need help and kept paying my salary just to be available.

That said, if you're a lowly paid shit kicker that would seem wrong as typically a new junior job they want to to start ASAP and wouldn't wait for 3 months. So that could hurt your ability to find new work.