How to modify a button using its name as a variable

Question

So I'm trying to figure out how to change a buttons text based on having the buttons name as a variable. I already know how to change a buttons text by hardcoding its name like Button1.Text = "hello";

But I want to be able to do something like storing Button1's name in a string called value and then use that string to modify the buttons text so i can use a method for several buttons instead of pasting a huge similar block of code in every buttons method, I just cant seem to figure out how. Any ideas would help.

private void Button3_Click(object sender, EventArgs e)
        {
            Button obj = sender as Button;
            string buttonname = obj.Name;
            boxfill(buttonname);
        }
private void Button4_Click(object sender, EventArgs e)
        {
            Button obj = sender as Button;
            string buttonname = obj.Name;
            boxfill(buttonname);
        }
private void Button5_Click(object sender, EventArgs e)
        {
            Button obj = sender as Button;
            string buttonname = obj.Name;
            boxfill(buttonname);
        }
// this will send the buttons name to boxfill

private void boxfill(string value)
        {
            // will ideally change the button named with value's
            // text to "x"
        }