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 first cousin's grandsons are your first cousins, twice removed.
The child of your first cousin, once removed, is your first cousin, twice removed, if your first cousin once removed is the child of your first cousin. If your first cousin once removed in the first cousin of one of your parents, the child is your second cousin.
If your first cousin once removed is your parent's first cousin, then your first cousin once removed is your child's first cousin twice removed.If your first cousin once removed is the child of your first cousin, then your first cousin once removed is your child's second cousin.
Let's take this one step at a time to make it clear. Your parent and your gran's sister's son are first cousins. That should be obvious. When a first cousin has a child, that child to the first cousin is a "first cousin, once removed". (The "once removed" part indicates that the relationship is a different generational level.) In short, your gran's sister's son is your "first cousin, once removed".
Your parent's first cousin is your first cousin, once removed. Your parent's first cousin, once removed is your first cousin, twice removed. Your parent's second cousin is your second cousin, once removed. Your parent's second cousin, once removed is your second cousin, twice removed.
There are two ways you can have a first cousin, once removed.If your first cousin, once removed, is the first cousin of one of your parents, then the daughter is your second cousin.If your first cousin, once removed, is the child of your first cousin, then the daughter is your first cousin, twice removed.
There is no such thing as any kind of cousin removed by any amount of percent. "First cousin 23 percent removed" is simply nonsense. You could have a first cousin twice removed, or three times removed or 23 times removed. You first cousin is the child of you aunt or uncle. You first cousin once removed is the child of your first cousins, three times removed is the grandchild, etc. But remember that you are the first cousin once removed to the child of your first cousin. So you also can have a first cousin once removed who is the first cousin of your parent; a first cousin twice removed who is your grandparent's first cousin, etc.
The daughter of your first cousin once removed is your first cousin twice removed, or your second cousin, depending on which generation your are in compared to your first cousin once removed.
Your grandmother's second cousin is also your second cousin, but twice removed. The twice removed means the number of generations between you. Your mother's first cousin is your first cousin once removed. If your mother's first cousin has children, they are your second cousins.
Your first cousin's child is your first cousin once removed.
Your first cousin's child is your first cousin once removed (one generation removed).Your first cousin's grandchild is your first cousin twice removed.
The son of your First Cousin Once Removed, is your First Cousin Twice Removed.