There are no restrictions or laws that would make it a problem to date, or even marry, a third cousin.
Your grandmother's third cousin is your third cousin, twice removed.
Your husband's third cousin is just considered your third cousin through marriage.
The son of your mother's third cousin is your fourth cousin.
Third cousin twice removed.
First cousins' grandchildren are third cousins to each other.
You can date anyone you want, whether third cousin or first cousin. Going beyond simply dating may raise issues with a first cousin, but not with a third cousin.
A third cousin is just that, a third cousin, there is no other name for the relationship.
Your father's first cousin's child is your second cousin. Your father's second cousin's child is your third cousin. Your father's third cousin's child is your fourth cousin. and so on.
Your grandson's third cousin is your first cousin twice removed, and you are hers. You count from the cousin who is closest in generation to the common ancestor in determining what kind of cousin relationship applies.
Your 4th cousin
third cousin.
Your first cousin's child is your first cousin's, once removed. Your second cousin's child is your second cousin, once removed. Your third cousin's child is your third cousin, once removed.