I believe these are requirements, that each of the separate teams working on these projects have to use wholly different suppliers to also minimize the chance that the same external factors negatively influence the system.

Possibly in order to use three different architectures to avoid any nasty arch-specific bugs

Also, different compilers:

Each PFC channel contains three dissimilar processor lanes, and software from Ada source code using three different Ada compilers to provide triple dissimilarity.

Now that is impressive.