Your great uncle (by dictionary definition) would be your grandparent's brother. His great uncle would be his grandparent's sibling. So it would work out to be your great great great uncle (3rd great uncle).
Your relation to your mums uncle is GREAT uncle. You're welcome! 
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
Your uncle is your children's great uncle and they are his great nieces and great nephews.
Your uncle is your daughters great uncle.
Your mother's uncle is your great uncle.
He would be his great great great uncle
Great Great Uncle He would be your Great, Great Uncle. You are his Great, Great Niece or Nephew.
Yes, he is their great uncle.
grand uncle or great uncle
She is his great grandniece.
Your great-great uncle.
Your granddaughter is your uncle's great great niece. He is her great great uncle, or some would say great grand uncle.
Your great grandfather's brother is your great uncle and your grandchild's great-great-great uncle
your the great great aunt or uncle
Your grandfather's brother is your great uncle and your parent's uncle.
You are aunt or uncle to your nephew, great aunt or great uncle to your nephew's son, and great great aunt or great great uncle to your nephew's grandchild.
Your aunt's great uncle is also your parent's great uncle. So 2Great Uncle would be one way of naming them.
You are great aunt or great uncle to your niece's daughter and great great aunt or great great uncle to her grandchild.
He is your great-nephew, and you are his great aunt or uncle.
You are their great aunt/uncle.
Great great aunt/uncle. To your niece, you are the aunt/uncle, to her child you would be the great aunt/uncle. So you would then be the great great aunt/uncle of her child's child (her grandson)
You are an aunt or uncle (depending on your sex) to your nephew, and a great aunt or great uncle to his children.
Your mother's brother is you uncle and your daughter's great uncle. Your daughter is his great niece.
Your great uncle! (no blood relation there, but still your uncle:)
You would be his Great Uncle and he would be your Great Nephew.