My 3-node serial crossover bus works but I don't understand why
There many &quot;sub-questions&quot; in your question, and I'm not going to try to answer them all individually. Instead, I'll explain my hypothesis, and I think you will find that this answers your various sub-questions, just not in the way that you asked them. :-) So, after doing some research, and from previous knowledge of RS-485 and RS-422, here is my &quot;educated guess&quot; about what is happening.

That DT-5019 USB-to-RS-485/RS-422 adapter advertises &quot;Automatic send data control. Auto detection of data speed; Zero delay automatic transmission; Automatic hand-shaking protocol supported&quot; but without enough details to explain how it works, or even exactly what those last two terms mean. It supports 2-wire (half-duplex) and 4-wire (full-duplex) connections, apparently without requiring any hardware (e.g. jumper) selection / switching.
To get a full understanding of how it works, which would help you try to understand the behaviour where you don't expect data sent from Node 2 to be received by Node 3, you would need to get more details (ideally a schematic diagram and &quot;theory of operation&quot;) from the vendor or manufacturer. Personally I doubt that you will get the necessary level of information. That is where some &quot;educated guesses&quot; become necessary.
I did find this &quot;Word&quot; document on the original Chinese manufacturer's website, which contains some Chinese &amp; English text and diagrams, although it still does not explain exactly how the device supports both 2-wire and 4-wire operation.
On the Amazon seller's page which you linked, there is this diagram, which appears to be an edited version of some from the &quot;Word&quot; document which I mentioned:

The upper part is what they describe as RS-422 (I disagree that standard RS-422 would apply for one of the diagrams, but that's a different discussion) and these are the 4-wire connections; the lower part shows 2-wire RS-485 connections. Personally, I suspect that the adapter uses RS-485 transceivers (as these are backward-compatible with the RS-422 standard) even when using the connections labelled &quot;RS-422&quot; in the diagram.
Here is a hypothesis which seems to fit the behaviour which you observed (i.e. Node 2 received a transmission via which you believed to be its &quot;transmit connections&quot;) and would allow for both 2-wire and 4-wire Multi-Drop (1 Tx, more than 1 Rx) and also Multi-Point (more than 1 Tx, 1 or more Rx) usage:

I suspect that the connection pair marked &quot;T/R&quot; (+/-) always switches between Transmit and Receive modes, even with 4-wire connections. That pair would have to switch anyway, to allow half-duplex operation with 2-wire connections (standard RS-485 half-duplex).

I suspect that the connection pair marked &quot;RXD&quot; (+/-) are always operating in Receive mode, whether there is a 4-wire connection (when obviously they are needed) or a 2-wire connection (when they are not used)
In this way, what seems to be automatic switching between 4-wire and 2-wire modes, is actually just one behaviour of the adapter, which works in both of those cases.


This is why, when you transmitted using the Blue / White(Blue) pair from Node 2 (connected to its T/R connections) the message was received by Node 1 via its &quot;RXD&quot; connections (as expected) and also by Node 3 using its &quot;T/R&quot; connections. Those &quot;T/R&quot; connections on Node 3 would be in Receive mode for a normal 2-wire RS-485 configuration, and I suspect they remain in Receive mode when that node is not actively transmitting.
You might be able to use a simpler approach. You said that:

Modbus wants a 'Master/Slave' setup, with Tx going to multiple Rx of the slave devices

The Modbus &quot;Master/Slave&quot; setup is a function of the protocol, but doesn't mean that the Master device has any different RS-485 connection than the Slaves. Modbus-over-serial is typically implemented using a standard 2-wire RS-485 bus (although a 4-wire connection could be used). Since the Modbus protocol is itself half-duplex, there is no need for a hardware interface which is full-duplex (e.g. the 4-wire connections shown in the diagram above) as long as your RS-485 devices stop driving the bus quickly after transmitting.

BACnet appears to want everything on a completely shared bus

That is exactly what Modbus uses too.

all the Tx goes to the Tx of all devices

There are some poor BACnet MS/TP (MSTP) diagrams on the internet, which show the two wires of RS-485 labelled as &quot;Tx&quot; and &quot;Rx&quot;, which is not true at all. I suspect you have been misled by that. In RS-485 there is no &quot;Tx&quot; wire.
I have only used Modbus, but I researched BACnet in preparation for this answer. I didn't find anything that indicated a need for different wiring between Modbus-over-serial using RS-485 or BACnet MS/TP over RS-485.
Therefore overall, I suggest that you consider simply using a 2-wire implementation of the RS-485 wiring, as this is standard for Modbus and also seems to be standard for BACnet MS/TP.
Finally - normal RS-485 requirements need to be considered e.g. cable type, ground (common) connections, termination at the ends of the bus (except in very rare circumstances) and the consideration of failsafe-biasing. Unfortunately I couldn't find any information about whether that USB adapter uses transceivers with built-in failsafe-biasing or not. If you were able to easily open one of these adapters and if the (likely RS-485) transceivers still had their manufacturer's part number, then it would be possible for you to research their specification.