C#

// assume all three NumericUpDowns have "Minimum" and "Maximum"
// values set to: Decimal.MinValue, and Decimal.MaxValue
private Func<decimal, decimal, decimal> PE;

private decimal x, y;

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    x = numericUpDown1.Value;
    y = numericUpDown2.Value;

    switch (comboBox1.SelectedIndex)
    {
        case 0: // multiply
            PE = (d1, d2) => d1*d2;
            break;
        case 1: // divide
            // probably better to handle divisor == 0 here ?
            PE = (d1, d2) => d1/ d2;
            break;
        case 2: // power
            // need to handle case of raising to the 0th. power here ?
            // as is: zero to the zeroth. power will be returned as #1 !
            PE = (d1, d2) => (decimal) Math.Pow((double) d1, (double) d2);
            break;
        default:
            // throw error here during development
            break;
    }

    try { numericUpDown3.Value = PE(x, y); }
        // handle System.DivideByZero exception
        catch (DivideByZeroException dZero) { }
        // handle System.Overflow exception: Decimal.MinValue or Decimal MaxValue
        catch (OverflowException oFlow) {}
}