Actually it came into service in the fall of 1917. However, due to a series a structural failures, production of the airplane was limited and only 320 or so were ever manufactured. It was replaced by the Fokker D.VII biplane after von Richthofen's death.
It was also unable to keep up with the faster Spads and S.E. VAs and that were simply faster and could attack and fly away to set up another attack it simply could not respond to.
That's why it was replaced by the Fokker D.VII. The only plane the Triplane could compete with was the Sopwith Camel and maybe even French Nieuports. It wouldn't be able to fight much else despite its advantages. Tripes tend to be VERY DRAGGY machines.
The Nieuports were on their way out by then. They were already being replaced by the Spad 7. The Fokker D.VII is generally thought of as the best plane of the war.
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