You can't trim a line to a 3D element so I use the Extract Edges (XEDGES) command on the 3D element. The XEDGES command adds lines to all the edges of the box that need deleted afterwards so before starting the command change the color in properties to a color not used in the drawing to allow for easy selection with the QSELECT command to delete them once you have trimmed your line.
1. Change color in Properties to something unique to drawing
2. Extract Edges of the box
3. Start the Trim command and hit Enter to <select all> the elements in the drawing. Select the line to trim.
4. Start QSELECT and change color to the one used with XEDGES. Delete all colored items.
