Imo, Die Hard 1 is the only one where he is truly a regular cop in the wrong place at the wrong time.
MartyMcFly's points about Die Hard 2 are true, and so are Ebonic Plague's points about Die Hard 3.
I guess you have to ask youself, for a true Die Hard sequel to have even been written, the question would need to be, how does a regular cop get put in the wrong place at the wrong time again? In Die Hard 1, the attack was probably his first of this kinda experience and we all saw how worried and fragile he was, despite his abilities. By Die Hard 2, if theres one difference between McClane in this one and the first one, its that hes already gone through that immense experience. That however shouldn't make him 'Sherlock Holmes' as has just been said. I think the writers just went wild and thought they could make McClane more like Schwarzenegger in 'physical effectiveness', and Colombo in 'detective work' based on what he gained from the first film.
With Die Hard 3, he survives alotta shyt he shouldn't have survived, even someone else I watched it with pointed out how random it was that Samuel L Jackson just happened to be driving past as he gets thrown out of the water tunnel lol. I think the problem is that it isn't written as well as it could've been, despite being a great script. Hes hungover and that is brought up throughout the film, plus has been depressed in the years leading up to the film. This is really the worst time for him to be 'surviving everything'. But I get Marty's point about 3. He may physically be 'Schwarzenegger' in 3, but his mind isn't as 'Sherlock Holmes' as in 2. In 3, he really should've died once they got out of the elevator, but got lucky cos he saw Otto's badge that just happened to belong to his colleague, then had the advantage of surprise as a result. Still should've taken atleast one bullet though considering they all already had the intention of killing him. Overall, luck is on his side throughout the film almost to make up for his mental state. He also had a non-insane Zeus, a whole police department, plus that truck driver to help him out.