In my honest opinion i think that in Star Trek 3 should be based on The Next Generation..i love the NG charcters more then the original Star Tek Charcters. I won't fancast every character. Here is my fancast
Captain Jean Picard/ Christopher Eccelson
It is so very hard to for another actor besides Patrick Stewart portray Captain Jean Picard but the best choice for Captain Jean Picard is Christopher Eccelson, he is a really good actor would do a fantastic job as Jean Picard.
Data/ Tom Hiddlson
I freaking love Tom as a Actor and in my honest opinion he would do a fantastic job as Data. He really does look like Data and is a fantastic actor..
Lieutenant Geordi La Forge/ Chadwick Boseman
I know many people would say that Chadwick would do a great job as Black Panther but honestly i feel it would do a great job as Gordi La Forge.
Henry Cavil/ Commander Willaim Riker
Everyone knows who Henry Cavil is and i hope he gets more roles besides Superman. I think Henry would do a fantastic job as Willaim Riker and he looks like Riker to.
Doctor Beverly Crusher/ Mary Elizabeth Winstead
I really like Mary Elzabeth Winstead as a actress and she would do a great job as Beverly Crusher.
General Worf/ Jason Momoma
I think Jason is a good actor despite some of the movies he has been in and would do a really good job as General Wolf..
Villain:
Q/ John Barrowman
Q is a great charcter in Star Trek: The Next Generation and would make a good villain for Star Trek 3. I love John Barrowman and would do a fantastic job as Q