Subtracting 3 form fields without continuing to minus their result when inputting other form field

Hello, please help me.

Is there a custom calculation script for this to happen? I have 3 fields to be subtracted from the first form field and when I input other form filled they keep subtracting from each other (only the 3 form filled)

this is my calculation script:

var a = this.getField("3").value;

var b = this.getField("2").value;

var c = this.getField("1").value;

event.value = a;

if (a !=="" && b !=="") {event.value = a - b;}

if (a !=="" && c !=="") {event.value = a - c;} ;

;

So the final result of their difference would be on Field 3.

Please tell me where I went wrong on this one. I also tried setting their calculation order it just keeps messing up. Or is there a custom script for this to prevent from subtracting furthermore.

Thank you! I use adobe acrobat