You should make one boolean variable hasPassed=False.
Then as parse all TextBox.Text to Integers.
Following your logic, everything above here is superfluous. Use the boolean variable instead
If (Mynum1 + Mynum2 + Mynum3 > 10) And TxtboxCat.Text = "A" Then
hasPassed = true;
If (Mynum1 + Mynum2 + Mynum3 > 15) And TxtboxCat.Text = "B" Then
hasPassed = true;
If (Mynum1 + Mynum2 + Mynum3 > 18) And TxtboxCat.Text = "C" Then
hasPassed = true;
Then set the label based on the result.