They would be out of synch with the common ancestor by three generations. That makes the a first cousin three time removed.
3rd cousins
Your grandson and your sister's grandson are second cousins to each other.
Your great grandson and your husband's great niece are second cousins, once removed.
A great niece and a grandson of the same person are second cousins to each other.
Your daughter's son's son is your great grandson. Your daughter and your sister's son are first cousins. Your great grandson and your sister's son are first cousins, twice removed. Your great grandson and your sister's great grandchild are third cousins.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your grandmother's brother is your great uncle. You and your great uncle's grandchild are second cousins to each other. Your and your great uncle's great grandson are second cousins, once removed.
Second cousins.
Your grandson and your brother's grandson are second cousins to each other.
Your great aunt is the sister of your grandparent Her son's son is her grandson. Her grandson and you share a great grandparent and thus are Second Cousins to each other.
Your grandson and great grand son could be cousins,(first cousins once removed), they could be uncle and nephew, or they could be father and son. It depends on how they are related to your children. If your great grandson is the grandchild on one of your children and your grandson is the child of a different one of your children, they are first cousins, once removed. If your great grandson is the child of a sibling of your grandson, then they are uncle and nephew. But your great grandson could also be the son of your grandson. Then they are father and son
You and your great uncle's grandson are second cousins.