If "God" is a Trinity, where in the Bible does the word "God" refer to the Trinity? There may be places where the word is ambiguous.   What I am looking for is one example that must be all three.
The short answer is: nowhere.  But your question is misguided.

You cannot disprove the nature of God as Trinity by looking at how the NT authors use the word "God" (Θεός or Κύριος) because within a few decades after the revelation that Jesus was the 2nd member of the Trinity (as God the Son), the NT authors were still transitioning their terminology from Θεός and Κύριος ("God") to "Father".  The word "God" / "Lord" used to refer to Israel's God in the OT for at least 1,000 years.  It's understandable that for the sake of continuity, the NT authors were still using the same word "God" to mean God the Father who sent His Son when the sentence context needed it, while gradually reassigning "Lord" for Jesus, which is yet another sign of Trinitarian understanding driving the terminology change.  It was also logical to use new words for Jesus (such as Χριστός) and the Holy Ghost because both were newly revealed, bringing what was vaguely latent in the OT to be more explicit in the NT.

Thus the apostles were still in the very early stages of the Trinitarian formula development although they already understood God as Trinity shown by how they talked about worship, baptism, salvation, and eschatology because the roles of the Father, Son, and Holy Spirit are distinct in each of those areas.  Please don't confuse formula development (which has to do with words and definitions, like what your question implies) with apostolic understanding of the nature of God.  It's like Newton already understood the laws of mechanics before Einstein refined the formula.

For an example of an apostolic understanding of the Trinity and a sign of the transition, simply look at one of the earliest record of the baptism formula in Matthew 28:19: "In the name of the Father, the Son, and the Holy Spirit", which in Greek is εἰς τὸ ὄνομα τοῦ Πατρὸς καὶ τοῦ Υἱοῦ καὶ τοῦ Ἁγίου Πνεύματος.  Where is the word Θεός or Κύριος in that formula?  Nowhere!  But surely God has to do with baptism, don't you think?

Consider @curiousdannii pointing out 1 Cor 2:10-16.  This was an even earlier sign of apostolic understanding of the Trinity, since this letter was commonly dated to early 50s rather than Matthew's early 60s.  In this passage, the word "God" in verse 10 clearly shows how St. Paul had the Trinity in his mind, since within the same section he explained the how it was possible for humans to comprehend the mysterious nature of God as Trinity by exploring the relationship between God, the Spirit of God, and the mind of Christ.

Other examples of apostolic understanding of God as Trinity can be seen in @Geremia's answer.  I understand that these verses don't answer your question, but it's a good list showing how the apostles already conceived God as Trinity within a mere 20-30 years after Jesus returned to heaven from where He came.  Recent research has also validated how NT authors's understanding of God came exclusively from Jewish perspective of God's fulfilling His promises rather than too much influenced by Hellenistic ideas (as Philo was).  (Example: see this paper comparing John and Philo).  (As an aside, the Jewish philosopher Philo was also marginalized by their own people within the Rabbinic Tradition).  Therefore we can safely say that the apostolic Trinitarian understanding of God wasn't due to some pagan influence as your other question tried to establish.