Yes, you can activate up to 3 of the same structures on the board, by paying the activation costs of those you activate. What you can't do is activate the same structure more than once per round action.
There is no rules limitation aside from wrenches played to how many buildings you may activate in a single action. The base game only has 3 of each building in its token pool, but if an expansion added more later, there would not be a limit of up to 3 according to the current rules.
You can Build and activate any number of Structures with a single Action as long as you have sufficient Building points to pay for it.
A Building can only be Activated once per Action, but can be Activated multiple times in a single Round if you take the Build Action again.