Well, the camera does not actually magnify against any other reference, the 3x is not relevant on it's own, except relative to the camera's own "1x" if it's a zoom camera. But if you accept that, then you can say that magnifications are multiplicative - the size of the object will be 21x bigger than if you were to zoom out to "1x" on your camera and point it at it. A little bit more natural example is that if you look through a 7x scope behind a 10x scope, you have 70x of magnifcation (of likely a really, really bad picture though

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