People who get 10 year visitor visas have a very strong premise which is deemed to be 'permanent'...
- Visiting a grandchild (or a similar family member in the descendent
line) who lives in the UK
- Owning a business in the UK or having a reason to travel to the UK
for business
- Shopping and social events requiring the person to be in and out of the UK
frequently
- Owning a house in the UK that the person uses during the social season
- Owning a mooring place and a vessel that sails in annually
- Owning an aircraft that stops in the UK for social and sporting
events
- And so on
And along with the premise, the person has built up a history of performance over a long period of time, like 10 years or so. You have had 3 visas over the course of 8 years, but to have an application taken seriously you would need about a dozen over that time and all for the same reason.
Based upon what you wrote you will have difficulty establishing both premise and performance, so it's fair to be pessimistic about the outlook.
You can always apply however; it's simply a matter of checking the appropriate box and paying the fee. If they decide you do not qualify for a 10 year visa, they will work their way down the line looking for something you do qualify for until they get to the standard 6 month visa. If you qualify, you'll get the standard 6 month visa and they will keep the difference in fee.
Adding...
I forgot to include something important... In some consulates it is standard practice to automatically 'upgrade' an entry clearance to a longer duration, even if the applicant didn't ask for it. It's not really 'automatic', but if the ECO determines that an applicant has a qualifying premise and great performance history, she will assign the next higher duration as a matter of course; you don't need to do anything. They do not notify you either, if you get your entry clearance in the post and it has a longer duration than what you asked for, then congratulations, you got an 'automatic upgrade' (that isn't really automatic, and isn't really an upgrade).
