Checkboxes cannot be unchecked when used in JavaScript Calculation

Hello everyone,

I have got a problem with checkboxes which are used to calculate the value of a field with a few lines of JavaScript code. This is the code I am using at the moment (I set the output values of the checkboxes to 1):

if (getField("Checkbox1").value = 1){
     if(getField("Checkbox2").value = 1){
     if(getField("Checkbox3").value = 1){
     this.getField("Calculation").value = 1.1

     }

     }

}

After that I want to set the value to 1.0 if only 2 boxes are checked and then to 0.9 when only one is checked.

The calculation works fine but if I try to uncheck a checkbox after the calculation it will always re-check it as soon as I input another number in a random other field on the document. So I open the document and check all 3 boxes; calculation of value 1.1 works fine; I uncheck one of the boxes; input a number somewhere else on the document and the formerly unchecked box is now checked again without me doing it.

Can anyone help with this issue?