The real key to your problem is the opening statement of your post; "... What am I doing wronf. For 3 years..."
Thing is one shouldn't plant cucumbers in the same spot one year after another, nor should they be planted where tomatoes have been grown and vice-versa.
Cucumbers (and tomatoes) are subject to black spot and other fungii... quite susceptible. Try growing any variety of beans in that soil next year (they replace the depleted nitrogen for one thing). Next, plant you cucumbers somewhere they haven't been grown for at least one year... two is better... Sorry about this year, although, if it were here in the U.S. a greenhouse might have a nice, sturdy cucumber plant or two to try as suggested...