int n1 = 0;
int n2 = 0;
int n3 = 0;
n1 = Convert.ToInt32(txtNum1.Text);
n2 = Convert.ToInt32(txtNum2.Text);
n3 = Convert.ToInt32(txtNum3.Text);
if ((n1 > n2) && (n2 > n3))
lblShow.Text = n3 + "-" + n2 + "-" + n1;
else if ((n1 < n2) && (n1 > n3))
lblShow.Text = n3 + "-" + n1 + "-" + n2;
else if ((n2 > n1) && (n2 < n3))
lblShow.Text = n1 + "-" + n2 + "-" + n3;
else if ((n2 < n3) && (n1 > n3))
lblShow.Text = n2 + "-" + n3 + "-" + n1;
else if ((n3 > n1) && (n3 < n2))
lblShow.Text = n1 + "-" + n3 + "-" + n2;
else if ((n2 < n1) && (n1 < n3))
lblShow.Text = n2 + "-" + n1 + "-" + n3;
acho que é isso ai.