Dear Vincent,
Disclaimer: I've never used Telemac3D.
In TELEMAC2D I,J represent the exact same as the components of the velocity multiplied by depth in that node ( [U*Depth] and [V*Depth]), making an integration along a line and taking into account the direction of the flow, gives the net "flow discharge" through a section.
Recalling that Tele3D works in a plane system ...Wouldn't I and J would represent the exact same (component*vertical distance*length ponderation based on distance to plane above and below) for each plane?
Integrating all vertical (sum operation, taking into account direction of vectors and cross section), and horizontal too(multiplying for adjacent distance in horizontal plane) would result in the net flow.
Regards,
José D.