I realise posters have been kind enough to explain why Maurits and Bernhard will lose their place in succession but I'm not sure I understand.
You say that you must be related by three degrees? Surely currently then with Beatrix as Queen the three degrees are her children, grandchildren and sister. Why do her nephews still have a place?
And when W-A ascends why is the three degrees not his children, brother and nieces and nephew? Why is Margriet still included?
The degrees of relation don't work quite like that.
Those currently in the succession line are the Queen's relations in three degrees:
Beatrix -> Margriet (sister through royal parent) -> Maurits and Bernhard (sister's sons)
Beatrix -> Willem-Alexander (son) -> Amalia, Alexia, Ariane (son's daughters)
Beatrix -> Constantijn (son) -> Eloise, Claus-Casimir and Eleonore (son's children)
Three degrees of relations from Willem-Alexander include only his own daughters, his brother, his brother's children and his aunt.
Willem Alexander -> Amalia, Alexia, Ariane (daughters)
Willem-Alexander -> Constantijn -> Eloise, Claus-Casimir and Eleonore (brother's children)
William-Alexander -> Queen Beatrix (royal parent) -> Princess Margriet (mother's royal sister)
Bernhard and Maurits will be excluded because they are Willem-Alexander's relations in fourth degree:
William-Alexander -> Queen Beatrix (mother) -> Princess Margriet (mother's sister) ->
Prince Bernhard and Prince Maurits
In the same way, Maurits and Bernhard's children are currently not in the succession line because they are Queen Beatrix's fourth degree relations:
Beatrix -> Margriet (sister) -> Maurits (sister's sons) ->
Anna, Lucas and Felicia (the children of the sisters son)
Beatrix -> Margriet (sister) -> Bernhard (sister's sons) ->
Isabella, Samuel and Benjamin
Theoretically, if Amalia had a child during Queen Beatrix's reign, the child wouldn't have been in the Line of Succession as well, despite being heir apparent (to the heir apparent of the Heir Apparent)