I looked up the "现代汉语词典".when 了 is used as an auxiliary verb
, it has two main functions:
According to the dictionary,了 in the sentence 1 satisfies the grammar point 1, like your understanding, it marks the completion of an action.了 in the sentences 2 and 3 satisfy the grammar point 2, like your understanding, it marks a change of state.
Tonight we will invite them to eat Beijing roast duck. (Yuck!)
In the evening we invited them to eat Beijing roast duck. (Yuck!)
Next month I return to the States. (Kiss me while you still can!)
You should go to customs now to complete the formalities.