C# winform minus 2 textboxs show answer in 3rd in reverse order

Question

This should be easy to fix but I cant fig it out.
I have 3 textboxes one below the other as a example. I tried textbox change

It works fine if I enter in order but if the user is out of order the math is wrong.
Is there a work around?

TXT1 - TXT2 = Correct answer
TXT2 - TXT1 = Wrong answer

private void CalcTotal(object sender, EventArgs e)
        {

            if (int.TryParse(txt_BackedOut.Text, out int i) && int.TryParse(txt_TOTAL.Text, out int j) && int.TryParse(txt_Online.Text, out int t))
                textBox1.Text = (j - i - t).ToString();

        }

Answer 1

Needs some polishing up, see if this might work for you.

public partial class Form1 : Form  
{  
    private readonly List<TextBox> _textBoxes = new List<TextBox>();  
    public Form1()  
    {  
        InitializeComponent();  
    }  
  
    private void Form1_Load(object sender, EventArgs e)  
    {  
        _textBoxes.Add(textBox1);  
        _textBoxes.Add(textBox2);  
        _textBoxes.Add(textBox3);  
  
        _textBoxes.ForEach(textbox => textbox.TextChanged += OnTextChanged);  
    }  
  
    private void OnTextChanged(object sender, EventArgs e)  
    {  
        if (_textBoxes.All(texbox => int.TryParse(texbox.Text, out _)))  
        {  
            int.TryParse(textBox1.Text, out var v1);  
            int.TryParse(textBox2.Text, out var v2);  
            int.TryParse(textBox3.Text, out var v3);  
  
            textBox4.Text = (v1 + v2 - v3).ToString();  
  
        }  
        else  
        {  
            textBox4.Text = "";  
        }  
    }  
}  

Answer 2

Do you mean if you enter values in your textboxes in different order? (i.e. BackedOut, Total, Online -> Online, Total, BackedOut)

Answer 3

You mean like?:

textBox1.Text = (-j - i - t).ToString();