您似乎错过了关闭button2.setOnClickListener上的花括号}，并将button3.setOnClickListener放入button 2单击侦听器中。只需关闭button 2单击侦听器中的startActivity（intent）后的括号，即可正常工作。

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main2)

val button2 = findViewById<Button>(R.id.button2)
val button3= findViewById<Button>(R.id.button3)

button2.setOnClickListener {
    val intent = Intent(this, MainActivity::class.java)
    startActivity(intent)
}

button3.setOnClickListener {
     val intent = Intent(this, Main3Activity::class.java)
     startActivity(intent)
}