You need to set the selected tab like so
private void button1_Click(object sender, EventArgs e)
{
tabControl1.SelectedTab = tabPage1;
}
private void button2_Click(object sender, EventArgs e)
{
tabControl1.SelectedTab = tabPage2;
}
private void button3_Click(object sender, EventArgs e)
{
tabControl1.SelectedTab = tabPage3;
}