Since signal moves in cable at about 5ns/m then 3ns corresponds to about 0.6m. Any cable longer than that (including all internal connections of the source and receiver) will behave like transmission line (might have reflections).
It’s not sufficient to have the cable acting as a transmission-line. The inevitable reflections must be avoided at the receiver by selecting optimum lengths. This is why I recommend always at least 1.25m length or 6 inches, nothing in-between that. See this white-paper I wrote years ago on the subject:
https://positive-feedback.com/Issue14/spdif.htm
Steve N.
Empirical Audio