There are a large number of ultra thin cameras that fit that description.
Since you did not include a target price, I would suggest the Canon SD200.
The physical size is 3.4 x 2.1 x 0.8 inches.
Link to specifications is here:
http://www.dpreview.com/reviews/specs/Canon/canon_sd200.asp
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I would appreciate it if someone could recommend a 3 megapixel camera for snapshots that's small enough to fit unobtrusively in a shirt pocket. Thanks.

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