Can I sell my computer that I paid for on Hire purchase for 3 years?

If you are effectively buying it over time (so it is cheaper than buying it normally) then once you've paid all the payments I'd say it is yours from what you've said. They state you can dispose of it - if it works, so long as there is no work stuff on it and it is like any other computer, I see no reason why you can't sell it. But if you are unsure then contact the company.