Homemade ethernet hub not working with three computers
This should not work because it's not a hub.

The reason why it works if you have only two devices plugged in is that they can auto-negotiate which pair to use in each direction. When you have three devices plugged in "direction" is no longer well-defined.

Back in the day you could just wire devices together in a line without hubs using 10BASE2 cable, although it was slow and stopped working if you forgot the terminators.