What do these three things have in commmon?
A sandwich shop, a computer and a casino.
— from The Puzzling World of Winston Breen
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Could it be
they all have chips? A sandwich shop has a chip card reader (and potato chips, thanks @deep thought), a computer has computer chips, and a casino has poker chips.
I know this already has an accepted answer, but how about:
Virus - spoiled food (i.e. potentially romaine lettuce in the US in Nov. and Dec.), a computer virus (malware, Trojan, etc.), the "Lady-of-the-Night" sorta virus (well, you know ;-P)
The letter C (works with O too). A sandwich shop, a computer and a casino.