Brief Answer:
Not an issue

Detailed Answer:
Hello XXXXXXX

Thanks for the query

The risks of having a medically challenged child has only been defined for first and second degree relatives. Marrying your third cousin will carry the same risks as marrying general population.
In other words the risk of having a medically challenged child when you marry a third cousin is equal to risk of having a medically challenged kid when you marry outside of your family, which is very minimal.
Even otherwise regular followups and scan can detect genetic abnormalities early

I wish you all the best

Regards

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