There are certain diseases that might manifest with a colored rim in the eye. However if is far more likely that this is a meaningless artifact.
For your own piece of mind I would recommend a visit to an pediatric opthamologist. He or she can examine her eye thoroughly and put your concerns totally to rest.
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This makes it even more likely in my mind that this is an artifact.
But as I said the only way to know for sure is to see the appropriate MD for an actual exam, not a guess over the internet.
3 is definitely old enough to get a good eye exam.
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