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You are right, there is a risk of metastability. But I would say that it is almost always the case when trying to sample external data.
Using several FF in series with no logic in the middle can help avoid metastability (care is needed during P&R).
As for using 3 FF, it's to make sure glitch are not detected as an edge. Of course the size of the glitch that can be avoided depends on the clock speed.
This circuit can not be used in all case, but is OK for simple case.