From your showing script and your question, I believe your goal is as follows.

• You want to check the cell "C3".
• When the value of cell "C3" is 5, you want to put "moyen" to the cell "D3".
• When the value of cell "C3" is 6, you want to put "Gros" to the cell "D3".
• When the value of cell "C3" is not 5 and 6, you want to put "faible" to the cell "D3".

Modification points:

• When you want to compare the values with the if statement, please use == and === instead of =.
• When the value of cell "C3" is 5, you want to put "moyen" to the cell "D3". But, in your script, it seems that when the value of cell "C3" is 3, you want to put "moyen" to the cell "D3".
• I think that feuille.getRange(3,4) can be used only one time.

When these points are reflected in your script, it becomes as follows.

Modified script:

function calculdunpoids() {
  var app = SpreadsheetApp
  var feuille = app.getActiveSpreadsheet().getActiveSheet()
  var cellule = feuille.getRange(3, 3).getValue();
  var destRange = feuille.getRange(3, 4);
  var value = "petit";
  if (cellule == 6) {
    value = "Gros";
  } else if (cellule == 5) {
    value = "moyen";
  }
  destRange.setValue(value);
}

References:

• Equality (==)
• Strict equality (===)