Since this is an assignment, you should be doing it and I'm not going to do it for you; but I'll give you some hints.
2018-10-13_130016.jpg
Insert a userform in the visual basic editor and add textboxes and a command button something like above.
In the above picture, there's a button on the right with the label Button which is used to bring up the floating box. It has a macro (asdf) in a standard code-module assigned to it:
Sub asdf()
UserForm1.Show False
End Sub
The bottom box is TextBox4, the three above it are TextBox1, TextBox2 and TextBox3
There's a commmand button to the right of them whose code (in the userform's code-module) is:
Private Sub CommandButton1_Click()
TextBox4.Value = Application.Sum(TextBox1.Value, TextBox2.Value, TextBox3.Value)
Range("A1") = TextBox1.Value
Range("B2") = TextBox2.Value
Range("C3") = TextBox3.Value
End Sub
The entire math behind the value in TextBox4 is the sum of the other three textboxes.
No checks are made that they're all numbers.
That's as far as I'm going.