I have three HDS 12s that are linked. The unit with the navionics card installed works fine. The other two show the contour lines but no depth numbers or other features like creek channels or depth shading. Is there any setting to change this or does each unit have to have a card?

1st be sure to choose Navionics on the 2 other units and if it's the case just go under map options-view-settings and be sure you got the same settings under view and settings

Each unit does not have to have their own card if they are linked and it is a newer Navionics card. It sounds to me as it is just a setting that might not be turned on. Let me see if I can get a video or screen shot of what settings to fix, but to m knowledge they both just need to be set to Navionics in the settings and it will share. Be sure that you turn on the fishNChip layer as well when if using a Platinum, Nav+ regions or Nav+ card.

Sometimes the unit down't recognize the Navionics in the "master" unit so this is why I suggest to choose Navionics under chart menu and then check all the settings; it'sd easy to do by looking on the "master" under view & options and get the other 2 on the same settings
I assume that they are lnk by ethernet not by NMEA2000?