Why am I reading 20V at the receptacles on one household branch circuit?
Check the power on the wires themselves coming into the first box. If you have low voltage there, then you have a problem in your wiring between the service panel and the first outlet and you'll probably have to pull new wire.

If the voltage on the wire itself coming in to the first box checks out, then check the connections in all three of the boxes that those receptacles are in. Maybe you have a bad connection in the first one, and it's affecting the other two that are "downstream" from it. If you're wiring through the receptacles, check the little metal tabs on the sides of the receptacles to make sure they aren't cracked or missing.

If you're using the press-in connections on the receptacles, try the screw down connections. Electrical connections must be tight. Also consider pig-tailing the connections with wire nuts inside the boxes, so that the electricity to the downstream boxes doesn't have to go through the receptacles at all. Unless this is a GFCI-protected circuit, in which case the downstream receptacles must be fed through the GFCI receptacle. But in that case I'd still prefer the screw-down posts to the press-in connections.

It's also possible that one or more of the receptacles themselves are just faulty (broken internally).