"Then there will be three stages."
Translation:Poi ci saranno tre palchi.
Without "ci" the sentence would mean "Then they will be three stages."
"Ci," in this case, means "there."
Quindi ci saranno tre palchi was marked wrong. Since quindi can mean "then" as well as "therefore" why is it not accepted? In the absence of context it is uncertain if this is an implied result or merely means "at that point" or implies "next" in a sequence of things.
http://www.differenzatra.it/differenza-tra-palco-e-palcoscenico/. Palco is any raised platform; whereas, a palcoscenico refers only to a theatrical stage upon which the performance takes place.