Some doctors might agree but there are probably some who also consider them very similar, or even the same.
If you asked me to see if I notice a difference between all three people, I would tell you that yes I do notice a difference, but the actual diagnosis is still very difficult to differentiate without taking into account each person's history.
But, I do see your point. Person #1 is basing the worry on true feelings instead of just thoughts alone. The person actually feels symptoms and so that person believes there is a serious disease that needs to be found. The thoughts are based on real symptoms.
However, to me, person #1 and person #3 could be the same person. Person #1 could just happen to be "in between" disorders. Person #1 has anxiety symptoms and goes searching for symptoms, and no matter what the doctors say, will still believe that they have an unknown illness. Person #3 has the same symptoms, but instead, has actually gotten around to finding a specific disorder that matches their symptoms. They could then move on and look for more symptoms and continue the pattern of person #1, until they then find another specific illness they believe they have, again, based on the physical symptoms, which is the same as person #1. I think personally, person #3 possibly would be OCD only if the person has a fear of ALS despite not having any symptoms. If they are having physical symptoms then it is hard to label that as OCD, because typically OCD thoughts are based in the mind of the person, and not of reality. Having physical symptoms can constitute as reality, so maybe person #3 is not OCD. But then again, it is also how you interpret the physical symptoms so who knows? I just think that if you can label someone OCD despite them having physical symptoms and thus, a basis for thinking they have ALS, that you should also be able to label someone with only physical symptoms who goes on and spends hours a day surfing the web as OCD as well.
Also, the definition of OCD involves more than just thoughts. It is any intrusive "thought, impulse, or image." It gets tricky if impulses can be considered OCD related. Is the urge to surf the web 2 hours a day an impulse? Does it cause significant distress? Or Is it a compulsion? What about the person that "feels" dirty, and then goes and carries out compulsions. Example: The people who feel compelled to wash their hands repeatedly, even when they are already clean. Surely, their hands *are* clean, but they still have the feeling that they aren't. Even though it's a feeling, it is still pretty much considered OCD. Then there are the people who actually feel clean, but they are worried of germs and avoid contact with door knobs. Well, then that is would everyone would call clear-cut OCD. But, can it also be health anxiety? Maybe the person doesn't want to get dirty because he doesn't want to panic about it and start experiencing any physical symptoms, which would then bring him into panic knowing that he will have to again go on a search to find out what is wrong with him, or which germ he got in contact with, due to the fact that he knows he is a hypochondriac, which is why a lot of people who are hypochondriacs would try to avoid caffeine..it brings out physical symptoms that they would have to analyze. Or how about the person that feels clean, but just went to dinner and now feels he has deadly food poisoning. Is this OCD or HA? I mean, the thought is based on a real life event..going to a restaurant and possibly getting food poisoning and their health possibly suffering as a result. It is just really cloudy and confusing to me as to what is OCD and what is HA sometimes, without considering the person's history, and even then it is very confusing and probably there is a lot of overlapping of disorders involved.
What if after person 2 realizes that they don't have HIV, they then start believing that they contracted botulism, followed by getting a symptom (tingling) and then also believing they have MS. It might sound like OCD, but, is it possible to have health OCD? Wouldn't this be considered HA? All their worries are health related. It gets complicated. Someone with OCD I would think would have numerous intrusive thoughts that are varied in nature, not just health related worries.
I just happen to believe that HA, or hypochronia, is basically just "HOCD" or Health OCD. A lot of people with OCD have probably had a health worry, and a lot of hypochrondiacs have probobly had some OCD related (non health) thoughts. Then again, you can also have OCD about a health related subject..it can be quite confusing determining them apart. But, again, a lot of people might disagree with me so I can't be sure.
So yes, I do notice a difference between all the cases, but a lot of them can be either/or which is why I have a hard time distinguishing between them and think they are closely related. But like you said, it is not so much about the diagnosis itself, because many people have more than one anxiety disorder. It is all about just getting treated for the anxiety. CBT will work for both HA and OCD.