A Macintosh (now usually just Mac) is one of a series of computers made by the Apple company. A mackintosh is a style of waterproof raincoat invented in the 1820s by Charles Macintosh (the k was added to the raincoat name almost immediately). A McIntosh is a type of red apple grown primarily in eastern Canada and the northeastern U.S.
Macintosh and McIntosh are proper nouns, meaning the first letter (along with the I in McIntosh) is capitalized. Mackintosh is a common noun, so it is not capitalized. The plurals are Macintoshes, mackintoshes, and McIntoshes.
Now you know.