Unforuntately you can't get 3375 colours, because you can't indepently set shadow/highlight mode for individual R,G,B channels.
The standard 3-bits per channel gets you 512 colours. Shadow mode gets you an additional 512 colours, but 64 of these are duplicates of the main 512, so you're only really adding 448 unique colours. Highlight mode is the same.
512 + 448 + 448 = 1408