In your previous posting you said that for more than 2 Sparks you also need to buy a very expensive switch.
That is not really true. Once you have 2 fast Ethernet ports, like DGX Spark has, you can interconnect any number of systems without using a switch.
In the simplest case, you just daisy chain the systems and you configure in Linux the Ethernet interfaces as bridges.
For better performance, you can close the chain into a ring, with an extra cable. In this case the Linux configuration is more complex, because you must do IP-level routing, preferably with a routing protocol like OSPF, to be able to double the throughput between 2 systems, by using both paths through the ring.
The only advantage of a switch is a greater throughput when there are 4 or more systems, which happens only when all the interconnected systems are attempting to communicate simultaneously, in which case having only 2 paths through a ring will serialize some of the transmissions.
When only 2 systems attempt to communicate with each other, a ring has double throughput in comparison with a switch. You need 2 switches to match the throughput that a ring has, as long as it does not become congested. Or you could use one double-sized switch, with the ports partitioned between 2 VLANs, with each DGX Spark connected to 2 ports, in different VLANs. Both 2 switches or a double-sized switch would greatly increase the price.
For only 3 systems, a switch is useless, as it is worse than connecting the systems in a triangle and configuring the IP addresses for point-to-point links (the configuration as bridges is needed only for 4 or more daisy-chained systems). The configuration with 3 DGX Sparks seems optimal from the PoV of the performance per dollar ratio.