Are 3 Xenons just too much for the mesh?
No - I can run a mesh with 9 devices fine.
All Xenons are within 1 meter from Argon.
I assume that this within a reasonably quiet (WiFi/Bluetooth) environment. Have you fitted external antennas and enabled the external antenna in the program on each device? Signal strength is improved greatly with an antenna (something like a TE connectivity 2118309-X) and it could be that this is your problem.
It might be there is a mis-aligned bootloader version - you could try ensuring that is v1.4.2
Could you share the code running on the Argon and the Xenons? Have you tried just putting Tinker on each one?