I think you have it reversed. 3 years is a typical full computer depreciation term.
Yes, it is. 3 years is a lifetime in "computer years". But what I am saying is that many companies (esp. small businesses) simply don't need to replace equipment until it falls apart. I've seen many small businesses running 5-10 year old equipment and computers. Sure, if they could they would probably get brand new equipment, but available cash is generally the issue. If it aint broke, don't fix it.
Of course, then you have the other side of that where many businesses can replace equipment on shorter time scales, and absolutely need top of the line, "latest and greatest" computing power. I guess it just depends!