Actually tendonopathy is generally just an umbrella term to cover all tendon issues. Although it is sometimes used to describe a tendon injury that refuses to heal. Tendonitis and tendonosis are a more specific diagnosis of differing, and specific, problems.
They've started to move away from terming it tendinitis as there's often no inflammation present with the pain/dysfunctional and iirc they also found that there's no direct correlation between inflammation and pain/dysfunction ie a perfectly healthy tendon can have inflammation much like you get DOMS in the muscles themselves
I guess to be more accurate, you would have tendinitis to begin with (eg from running a marathon) then it would likely become tendonopathy once the acute inflammation has gone (essentially the grey area), then progress to tendinosis as the tendon begins to degenerate