Responsibility of the land owner. You can't commission work on a neighbor's property. If they refuse to fix it, have a lawyer draw up a letter stating its failing and threatening your home. Home owner's insurance may be willing to get involved on your behalf (I.e have one of their lawyers send it).

3 feet?
I would not worry about it.

Can the owner not remove the wall if they so wish

I think it is the responsibility of the owner of the wall. Also if the wall crumbles into the other property or some damage is caused by the failure of the wall, the wall owner bears that cost. So if it is your wall I would fix before it gets more expensive and if it isn't your wall you could point it out to your neighbors and share your concerns.

The retaining wall between our property and our neighbors' property (about 3 feet high) is crumbling. Does anyone know if there is a general law in DC that the person that owns the property whose land is being "retained" - i.e., the higher property - is responsible for the maintenance (or vice-versa)? I know a response may be - find out whose properly the wall lies on, and that is the responsible party (fyi, don't know that answer, but could get a survey). But was trying to figure out if there was a different answer regarding the land that is being retained/protected. I would be happy to split the costs 50/50, but want to go in to the conversation with all information.