I think you are
the dot above a lowercase I or J.
I may seem small and useless
the point above the letter I is indeed small, and you'd recognise the letter even without the dot
Several letters but less than one
lowercase J also has this part of a letter in it
In fact I am not here, not at all
But I am near; up above.
not a single lowercase I or J in the riddle itself, a couple in the intro text though.
Good James made me famous
In Jesus's "sermon on the mount", the King James Bible translation famously uses the tiny dot to make a point: "For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled"
So when he humbles himself I appear.
This might also refer to "James" (or "Jesus") beginning with the uppercase letter J, which would have a dot if brought to lowercase.
I often carry three crosses
For without them I am a contorted falsehood.
The dot is often called a "tittle", which has three lowercase t's (crosses). If those are removed, what remains is "ile", an anagram of "lie"
Remove one stroke and I become what I am.
in cursive handwriting, removing a stroke from "tittle" gives you "little".