Although the only Graysons I'm familiar w/ are girls, I do like the name!
Of the 3 you listed, I think my favorite is Grayson Kristopher, but I would spell Christoper w/ CH instead of K - unless Kristopher is a family spelling or you're Greek.
So... Grayson Christopher.
I know you said "no comments", but I have to agree w/ others about the other 2 choices.
I've never been that fond of Graham because it reminds me of the cracker, but it's an okay name. I like Miller but not really w/ Graham... the M's are too close together.
If you're going w/ Jaxton, I'd just go ahead w/ the traditional Jackson which I really like, so you & your son don't have to go throughout his life spelling his name. (Or just go w/ the Jax - but please not Jaxton.)
Also, if Miller is the middle name for Jackson or Grayson, to me, it looks like two last names strung together - Jackson Miller, Grayson Miller... both sound like the names for a law firm.
Grayson Christopher
Christopher Grayson
Jackson Christopher
Christopher Jackson
Jax Christopher
Christopher Miller
Everyone will have an opinion, but, in the end, it's your baby's name! So go w/ what you like!