Contrary to what the name might suggest, Jenkins X doesn't have much in common with the well-known Jenkins CI system. It is backed by the same company, but the entire concepts of Jenkins and Jenkins X are totally different.
While the other two tools are purposefully small and self-contained, Jenkins X is a complex solution with many integrations and a broader scope. It supports the triggering of custom build tasks, making it look like a bridge between a classic CI/CD system and GitOps.