If the computer hasn't been used for three years, your passwords, etc. which might be stored on it should be out of date and useless. Nonetheless, change all your passwords TODAY and every six months or so.
Securely and completely destroying all data on the disk seems to be overkill. The top security method is to feed the drive into a metal chipper, turning it into small chips of metal. This is used for nuclear secrets, etc. I doubt if you have any of them.
The extent to which you need to protect/destroy the drive contents depends on the value of the information. If you have nothing but old e-mails and documents, and no information which could prove embarrassing or subject you to blackmail, then simply making the drive unusable should suffice. Most of the information which could be used for identity theft is probably available on Google anyway. My point is: "nuclear secret"-level destruction is not necessary. Who is going to go to the trouble of extracting data from a ruined drive, unless there is a gain greater than the cost of doing so? (The cost is very high - bit-by-bit examination under an electron microscope takes time and expensive equipment.)
However, if there is stuff on the drive which might be of interest to future generations, you might want to just put it away on a shelf with old photographs and other memorabilia. Some great-grandchild might find it interesting - (but by then, they might not have the technology to access data on such an antique.)
(The data on an unused drive will deteriorate over time, due to the effect of external magnetism, especially the earth's magnetic field. The drive might not be readable in 100 years. I have some drives I use to archive photographic data - I refresh them annually. So storing the drive for the far future may not be as successful as you might wish - comparable to old photographs deteriorating over time.)
Blowing the drive up, putting bullets through it, performing a 'DOD-level' erasure, and other such solutions are really not necessary. Just break off the cable connections, or break the case with a hammer. Unless you are the custodian of nuclear secrets or similar high-value data, no-one will waste time repairing the disk to read your old e-mail.