TFT, guessing the percentage of each of your categories would be difficult. There are several other reasons why some people are selling. From talking with many sellers, including FSBO (for sale by owners), I hear them saying that they do not have to sell. I think they are the majority by far. (one example is the vacant lot next to one I have listed. Basically, the lots are no different regarding type of lot, density, tree coverage, views, access, etc, yet the ForSaleByOwner (FSBO) is priced almost three times the price of the one I have actively listed. That FSBO seller has called me several times to check my price, but has never changed his price. He says that he doesn't have to sell.)
As for the "must sellers," the answer isn't so easy. Many of the must-sellers have more than one property listed, and when they sell one of the properties, they will likely pull the others off the market. This is what I hear from many of them. Some of these people may have between 2-5 properties currently available.
Then, there are other wishful, but over-priced, sellers who have pulled their listings off the market until it picks back up. So, when the market does begin to show a substantial increase in activity, look for these to be back on the market. Again, many of these are currently very overpriced based on the current market. They do not classify as have to sell and are wishful fishing.
I don't hear many sellers saying they fit into category number 2, though I am sure there are a few. In fact, I have been working with buyers who are buying for the opposite reason, though we are talking more "mid-term" rather than short term. (3-5 years) -- I know some people consider this long term, but I am more conservative.
There are a few who fall into your cat. 3, those who are tired of dealing with rental property, but those are very, very few.