Yeah it is a Known Known and those 3 distros have tried and true reliability. The term certified is what they call it “Certified to run on X” and “Compatibility CertifIcation” it was in response to OP asking if linux is used in corporate world. It is, and for larger operations it is the 3 you mention. Personally I think Ubuntu hasn’t made it into the Corporate Desktop apps like SUSE/REL because you install it and have a hairy hippo or faceted cougar head as the backdrop, just doesn’t sit well with CEO stuffed shirt types when looking for a professional software.